Water quality is one of the most important factors determining the success of aquaculture. Fish live entirely within the aquatic environment, and changes in water temperature, dissolved oxygen, pH, algae, water transparency, ammonia, nitrite, hydrogen sulfide, and other water quality parameters can directly or indirectly affect fish health, feeding, growth, reproduction, and survival.
This article provides a practical overview of the major aquaculture water quality factors that farmers need to understand, including water temperature, light and water color, water color and disease, water color management, dissolved oxygen (DO), water transparency, pH, ammonia nitrogen, nitrite, and hydrogen sulfide.
The information in this article is provided for reference only. Actual water quality management should be adjusted according to fish species, stocking density, pond conditions, climate, water source, and local farming practices.
1. Water Quality Is One of the Key Factors in Aquaculture Success
Water is the most important living medium for fish. It not only directly affects the fish themselves, but also influences the composition, abundance, and distribution of natural food organisms in the water.
A good aquatic environment is the physical foundation for healthy fish growth. Therefore, water used for aquaculture must meet the requirements of fish growth and development in terms of physical, chemical, and biological conditions.
In freshwater fish farming, water quality is one of the key factors determining aquaculture success or failure. Water provides fish and other cultured organisms with a three-dimensional environment in which they live, grow, reproduce, and interact with other organisms.
Of course, different fish species have different requirements for water quality and other physical and chemical parameters. Understanding the environmental requirements of the cultured species, as well as the relationships and interactions between different water quality factors, helps farmers take appropriate measures to improve the aquatic environment and increase production efficiency.
Different farmers may have different understandings of water quality, and therefore may use different management methods. Some methods may appear reasonable but produce poor results, while other methods that seem unconventional may work very well in a particular situation. A method that works effectively in one location may produce very different results somewhere else.
This is why site-specific management and adapting practices to local conditions are extremely important in aquaculture.
Water quality should therefore be viewed as a complete system rather than as a collection of independent numbers. For example, dissolved oxygen, water temperature, pH, algae, organic matter, ammonia, and nitrite are closely connected. Improving one parameter may also affect several others.
2. Water Temperature
Fish are ectothermic animals, meaning that their body temperature changes with the surrounding water temperature. Normally, the temperature of a fish’s body is very close to the temperature of the surrounding water, with a difference generally within approximately ±1°C.
Water temperature directly affects fish survival and growth. Therefore, anyone involved in aquaculture needs to understand seasonal and daily temperature changes and how temperature influences the aquatic environment.
In most situations, average water temperature during the daytime is lower than air temperature, while daytime water temperature is generally higher than nighttime water temperature.
An exception may occur during pond overturn or severe oxygen depletion events, when changes in the water column can cause unusual temperature patterns.
When water temperature is low, water becomes denser. Therefore, bottom water is generally colder than surface water, except when the surface freezes. When ice forms at 0°C, the water below the ice can remain around 4°C because water reaches its maximum density at approximately 4°C.
2.1 Characteristics of Water Temperature Changes
The temperature of surface water changes with season and air temperature.
Within a single day, water temperature is generally lowest shortly before sunrise and highest around 2:00–3:00 p.m.
Throughout the year, water temperature is generally lowest in January and February and highest in July and August.
During the hot summer season:
In water bodies approximately 2–3 meters deep, the temperature difference between the upper and lower layers is generally around 2°C.
In deeper reservoirs and lakes exceeding 10 meters in depth, the temperature difference between the upper and lower layers can be much greater.
2.2 Effects of Water Temperature on Fish
All organisms need to reach a certain temperature threshold before normal development and growth can begin. This threshold is sometimes referred to as the biological zero temperature.
Humans are warm-blooded animals, meaning that body temperature remains relatively stable regardless of seasonal temperature changes. Fish, however, are ectothermic animals, so their body temperature changes together with the surrounding environment. In aquaculture, water temperature is therefore closely related to fish body temperature.
As a result, water temperature directly affects the metabolic activity of fish.
Water temperature directly affects fish metabolism, feeding activity, digestion, and growth. Different fish species have their own suitable temperature ranges and optimum growth temperatures.
Within the optimum temperature range, fish metabolism generally increases, feeding activity becomes stronger, and growth accelerates. Under unsuitable temperature conditions, fish growth may slow down, abnormal behavior may occur, and extreme temperature conditions can even cause mortality.
For many common freshwater species in China, including silver carp, bighead carp, grass carp, black carp, common carp, crucian carp, and bream, the suitable growth temperature range is approximately 20–32°C, with an optimum growth range of approximately 25–30°C.
When water temperature falls below 15°C, feeding activity decreases and growth slows.
When water temperature rises above 32°C, feeding activity may also decrease.
A commonly used practical reference is:
- Below 8°C: Fish generally stop feeding.
- Below 10°C: Fish begin entering a relatively inactive or dormant state.
- 10–15°C: Feeding begins, but growth remains slow.
- 15–20°C: Appetite increases and growth becomes faster.
- 20–30°C: Growth is generally fastest.
- Above 30°C: Growth tends to slow again.
- 8–10°C: Fish have relatively low activity and oxygen consumption, making this temperature range suitable for transportation.
When water temperature changes suddenly, fish can experience severe stress, illness, or even mortality.
Therefore, particular attention should be paid to temperature differences when stocking fry and fingerlings. As a practical guideline, the temperature difference should preferably be kept within 3°C during stocking. Fish should be gradually acclimated by floating the transport bag in the receiving water before release, allowing the water temperatures to become closer.
For this reason, feeding rates should always be adjusted according to water temperature and its relationship with fish growth.
2.3 Water Temperature and Fish Reproduction
Temperature is closely related to broodstock conditioning, induced spawning, egg production, egg hatching, fry development, and fish transportation.
Water temperature has a particularly direct influence on gonadal development and the timing of spawning.
In northern and southern regions of China, the spawning season of many fish species can differ considerably. However, spawning generally begins when water temperature reaches approximately 18°C.
For many commonly cultured freshwater fish, when water temperature is between 18°C and 30°C and weather conditions remain sunny for several consecutive days, induced spawning, egg production, fertilization rates, and hatching rates are generally favorable.
When water temperature is below 16°C or above 32°C, induced spawning may become less effective or may fail completely.
Below 16°C, hatching rates may decrease.
Above 31°C, deformity and mortality rates may increase.
2.4 Water Temperature and Fish Diseases
Changes in water temperature are closely associated with the occurrence of fish diseases.
When water temperature rises, many pathogenic microorganisms reproduce more rapidly, increasing the risk of disease outbreaks.
For example, grass carp hemorrhagic disease generally occurs during periods when water temperature rises.
During high-temperature seasons, organic matter in pond water decomposes more rapidly. Parasites, bacteria, and other potentially harmful organisms also become more metabolically active and reproduce faster.
As these organisms increase, both water quality and sediment conditions can deteriorate, increasing the risk of various fish diseases.
Lower water temperatures can also be associated with certain diseases. For example, water mold disease and Ichthyophthirius multifiliis (Ich) are often associated with cooler periods in early spring.
Different fish diseases therefore have their own temperature ranges in which they are more likely to occur.
Temperature ranges commonly referenced in practical aquaculture management include:
- Hemorrhagic disease: above approximately 25°C
- Bacterial gill disease: approximately 28–35°C
- Ich: below approximately 25°C, with around 15°C considered a particularly favorable range
- Anchor worm: approximately 12–33°C
Rapid changes in water temperature can have a particularly strong effect on disease outbreaks.
For example, grass carp hemorrhagic disease may become more severe when water temperature fluctuates significantly.
Repeated sudden drops and increases in water temperature can also contribute to outbreaks of stress-related diseases in fry and fingerlings. Large temperature fluctuations can cause physiological stress because the fish are repeatedly exposed to sudden changes between cold and warm conditions.
Practical Temperature Differences During Fish Transfer
As a practical reference, the maximum recommended temperature differences during transfer are approximately:
- Fish fry: within 3°C
- Fingerlings: within 4°C
- Adult fish: within 5°C
2.5 Water Temperature and Material Cycling in Ponds
Water temperature directly affects the metabolic activity of bacteria and other aquatic organisms.
Within an optimum temperature range, bacteria and other aquatic organisms can grow and reproduce rapidly. At the same time, bacterial decomposition of organic matter into inorganic nutrients becomes more active.
These inorganic nutrients can then be absorbed by phytoplankton through photosynthesis, producing organic matter and supporting the reproduction of various natural food organisms in the pond.
2.6 Water Temperature and Dissolved Oxygen
Under certain conditions, dissolved oxygen (DO) decreases as water temperature increases.
At the same time, rising water temperature increases fish metabolism, causing fish to breathe faster and consume more oxygen. Other oxygen-consuming processes also become more active.
As a result, high water temperatures can significantly increase the risk of oxygen depletion, particularly during the hot summer season.
2.7 Water Temperature Management
Water temperature management methods should be adjusted according to the season.
Spring
In early spring, shallower water is generally preferred because shallow water warms more quickly.
After the ice melts in early spring, water levels can initially be kept relatively low so that sunlight can reach the bottom and help increase bottom-water temperature.
As air temperature gradually rises, water depth can then be increased progressively.
Summer
During summer, maintaining a deeper water column can help fish avoid excessive heat because water below approximately 1 meter can be significantly cooler than surface water.
Increasing water depth can also promote thermal stratification, creating a warmer upper layer and cooler lower layer.
Groundwater, well water, and spring water can be useful sources of cooler water. However, cold water should preferably pass through a sufficiently long flow path or be stored in a reservoir and exposed to sunlight before entering the pond, allowing its temperature to increase gradually.
Winter
During winter, maintaining a relatively high water level can help retain heat and maintain pond fertility.
Even when the pond surface freezes, the water approximately 50 cm below the ice can remain around 4°C, allowing warm-water fish species to survive the winter under suitable conditions.
3. Light and Water Color
Water color is a comprehensive indicator of the physical and chemical characteristics of an aquatic environment. It can provide useful information about overall water quality.
Scientific interpretation of water color requires observation and analysis of the algae present in the water.
Water color is closely related to light availability. When light conditions are favorable, aquatic biological activity and productivity can increase, helping maintain a more active aquatic ecosystem and supporting oxygen production.
At night, there is little or no sunlight, so the pond ecosystem enters a period when oxygen consumption continues but photosynthetic oxygen production stops.
For the same reason, cloudy days with weak light can create a potentially risky oxygen environment, particularly in ponds with high biological oxygen demand.
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The water color of an aquaculture pond is determined by a combination of dissolved substances, suspended particles, plankton, the sky, pond bottom, and surrounding environmental conditions.
Among these factors, plankton—especially phytoplankton—is one of the major contributors to water color.
Different species and concentrations of plankton cause pond water to display different colors and levels of turbidity.
Plankton can serve as direct natural food for filter-feeding fish and, importantly, phytoplankton are also major producers of dissolved oxygen through photosynthesis.
Water containing relatively high levels of calcium, magnesium, and iron salts may appear yellow-green.
Water containing large amounts of dissolved humic substances may appear brown.
Water with high concentrations of suspended sediment may appear yellowish and turbid.
An excessive number of freshwater snails may consume large amounts of algae and make the pond water less fertile.
Too many mussels or shellfish may increase water turbidity.
Large populations of small unwanted fish can also disturb the bottom sediment and increase turbidity.
Young green water is generally considered a desirable pond condition.
Overall, a good aquaculture pond water color is often described using the traditional characteristics of “fertile, active, fresh, and clear.”
Black pond water is a clear sign of aging or deteriorating water conditions. The darker the water becomes, the more severe the deterioration may be.
This type of water color may be associated with the accumulation of uneaten feed, feces, decomposing organic matter, and dead organisms.
When these materials accumulate and settle to the pond bottom, their decomposition consumes dissolved oxygen and may generate ammonia nitrogen, hydrogen sulfide, nitrite, and other harmful substances.
The bottom sediment may become black and foul-smelling, while fish immunity and resistance may decline.
As a result, fish become more vulnerable to pathogenic microorganisms, potentially leading to disease outbreaks and, under severe conditions, mass mortality.
4. Water Color and Fish Diseases
Two broad categories of water color are traditionally considered favorable for freshwater fish farming.
The first is green water, including yellow-green, brown-green, and oil-green water.
The second is brown water, including yellow-brown, reddish-brown, and green-brown water.
These water colors are generally associated with relatively abundant plankton, which can provide natural food that fish can readily consume and digest.
Unfavorable Water Colors
Potentially unfavorable water colors include:
- Black
- Dark brown
- Copper green
- Rust-colored water
- Red water accompanied by a foul or fishy odor
Black water is generally associated with excessive organic matter.
Copper-green water may indicate a high abundance of cyanobacteria.
Red water may be associated with certain dinoflagellates and other algae that are not readily consumed or digested by fish.
Such conditions may contribute to intestinal problems and an unstable, overly simple algal community.
They may also increase the risk of algal crashes.
The Double-Sided Nature of Algal Blooms
The appearance of an algal bloom has two sides.
On one hand, a bloom can indicate nutrient-rich water and provide fish with relatively abundant planktonic food.
On the other hand, this type of water condition can be difficult to maintain over a long period.
When weather conditions suddenly change and large quantities of algae die, water quality can deteriorate rapidly.
The water may turn black and develop a foul odor, and severe oxygen depletion can occur, potentially resulting in mass fish mortality.
5. Water Color Management in Aquaculture Ponds
5.1 Blue-Green Water — Cyanobacteria
Blue-green water may indicate excessive nutrient enrichment and a high abundance of cyanobacteria.
Possible management measures include increasing the proportion of filter-feeding fish such as silver carp, using appropriate treatments to control excessive algae, replacing part of the pond water, and rebuilding a more balanced phytoplankton community.
5.2 Black-Brown Water
When pond water becomes black-brown, feeding should be reduced or temporarily stopped.
Fresh water can be added, and aeration equipment should be operated to increase dissolved oxygen and accelerate the breakdown and transformation of harmful substances.
Chemical or biological water treatment methods may also be used where appropriate.
Filter-feeding fish such as silver carp may also help consume excess plankton.
5.3 Red Water
Possible management measures include adding or exchanging water, applying appropriate algae-control or decomposition treatments, and adjusting nutrient conditions with phosphorus- or potassium-containing fertilizers where appropriate.
5.4 Milky or Rice-Wash-Like Water
If the milky appearance is mainly caused by excessive rotifers, appropriate control measures may be considered.
If the water quality has deteriorated, phytoplankton can be restored.
If the pond is nutrient-deficient, appropriate fertilization may be necessary.
5.5 Thin or Nutrient-Poor Water
If excessive filamentous algae or “green moss” is present, it may need to be controlled.
Where the pond is genuinely nutrient-poor, appropriate organic fertilizer can be used to improve water fertility.
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6. Dissolved Oxygen in Aquaculture
Dissolved oxygen (DO) is essential for the survival and growth of aquatic animals.
Oxygen dissolved in water is called dissolved oxygen.
Fish live in water and exchange gases through their gills, so the amount of dissolved oxygen in the water directly affects their metabolism and physiological activity.
6.1 Sources of Dissolved Oxygen in Water
Oxygen enters water mainly through two pathways:
- Oxygen diffusion across the water-air interface
- Oxygen production through photosynthesis by aquatic plants and algae
Phytoplankton and other aquatic plants are important oxygen producers in productive ponds.
Under normal conditions without strong wind and wave action, oxygen diffusion from the atmosphere may contribute less than biological oxygen production in highly productive ponds.
The amount of dissolved oxygen in water is affected by water temperature, time of day, atmospheric pressure, wind, water movement, and other factors.
Photosynthesis by Algae
Photosynthesis by algae and aquatic plants is one of the major sources of oxygen in productive aquaculture ponds.
Oxygen Transfer from the Atmosphere
Oxygen can also enter water through atmospheric exchange.
Wind-generated waves, water circulation, flowing water, and mechanical aeration can all increase the contact area between air and water and promote oxygen transfer.
6.2 High and Low Dissolved Oxygen
A dissolved oxygen concentration of around 5–5.5 mg/L or higher is generally considered favorable for many aquaculture situations.
However, excessive oxygen supersaturation can also create a risk of gas bubble disease, particularly in fry and fingerlings.
As a practical management reference, dissolved oxygen should remain above approximately 5 mg/L for most of the day, while avoiding prolonged periods below approximately 3 mg/L.
When dissolved oxygen is sufficient, fish generally feed actively, digest feed efficiently, grow faster, and achieve better feed conversion.
When dissolved oxygen is too low, normal fish activity is impaired. Severe oxygen depletion can cause surface gasping, floating behavior, and mass mortality.
High dissolved oxygen generally supports a healthier aquatic ecosystem.
When oxygen is sufficient, microbial processes involved in nitrogen transformation can proceed more effectively. Ammonia can be oxidized through nitrification toward nitrite and nitrate, while algae can assimilate available nutrients and produce oxygen through photosynthesis.
Therefore, maintaining adequate dissolved oxygen is one of the foundations of good aquaculture water quality management.
High dissolved oxygen supports faster fish growth and better feed utilization.
As a practical target, maintaining an average dissolved oxygen concentration above approximately 4 mg/L for most of the day, with around 5 mg/L being a desirable level, can provide a favorable environment for many cultured fish.
When dissolved oxygen is low, fish growth slows, immunity may decline, and susceptibility to disease can increase.
Severe oxygen depletion can suppress feeding and eventually cause fish mortality.
Low dissolved oxygen can also disrupt energy flow and nutrient cycling in the pond, contributing to water quality deterioration.
For example, large-scale algae mortality can increase organic decomposition and oxygen consumption, while harmful reduced compounds such as hydrogen sulfide may accumulate.
However, excessive oxygen supersaturation can create a risk of gas bubble disease in fry and fingerlings.
For species such as silver carp, bighead carp, grass carp, and black carp, fish may begin to show surface gasping when dissolved oxygen falls to approximately 1 mg/L, while concentrations below approximately 0.4–0.6 mg/L can result in severe oxygen stress and mortality.
For common carp and crucian carp, reported critical oxygen concentrations may be lower, approximately 0.1–0.4 mg/L, depending on conditions.
In deep-water aquaculture systems, sudden rainfall during summer can sometimes cause serious fish mortality because of changes in thermal stratification.
Rainfall can rapidly cool the upper water layer and disturb the water column, causing oxygen-depleted bottom water to mix upward.
Organic matter and other oxygen-demanding substances from deeper layers may then become distributed throughout the pond.
As a result, dissolved oxygen levels throughout the water column can decline rapidly.
At first, fish may be able to avoid the low-oxygen layer. As mixing continues, however, they may be exposed to both low dissolved oxygen and other harmful substances, potentially resulting in severe stress or mortality.
6.3 Daily Changes in Dissolved Oxygen
Because photosynthesis by aquatic plants and phytoplankton depends on light intensity, dissolved oxygen also changes throughout the day.
Generally, dissolved oxygen is higher on sunny days than on cloudy days.
On sunny days, dissolved oxygen often reaches its highest level during the afternoon, and the upper water layer may become oxygen-saturated.
Dissolved oxygen is usually lowest around dawn, after aquatic organisms have consumed oxygen throughout the night while photosynthesis has stopped.
In ponds without sufficient mechanical aeration, this period can be associated with fish surfacing or “morning oxygen stress.”
Under low atmospheric pressure, calm weather, and stagnant water, dissolved oxygen tends to be lower.
Under higher atmospheric pressure, stronger wind, waves, and water movement, oxygen transfer into the water can increase.
In aquaculture ponds, approximately 80–90% of oxygen consumption may be associated with plankton, benthic organisms, and organic matter decomposition, while fish themselves may account for a smaller proportion of total oxygen consumption.
6.4 How Aeration Helps Maintain Aquaculture Water Quality
Dissolved oxygen is one of the most important parameters in aquaculture water quality management, but maintaining adequate oxygen is not simply a matter of measuring DO.
Oxygen demand changes continuously with fish biomass, water temperature, algae activity, organic matter, weather conditions, and time of day.
Mechanical aeration provides a practical way to supplement natural oxygen transfer and improve water movement. This is particularly important in intensive fish ponds, where fish biomass and organic loading can be much higher than in natural water bodies.
Different aeration systems are suitable for different pond conditions.
Paddle wheel aerators are widely used in pond aquaculture where strong horizontal water movement and oxygen transfer are required. For more information, see our [Paddle Wheel Aerator] product page.
Impeller aerators can provide both oxygen transfer and water circulation and may be suitable for deeper pond conditions. Farmers can compare their application with other systems on our [Impeller Aerator] product page.
Air jet aerators are another option for aquaculture systems requiring effective aeration and water movement. See our [Air Jet Aerator] product page for more information.
The choice of aeration equipment should consider pond depth, stocking density, oxygen demand, water circulation requirements, power availability, and the overall farming system.
Aeration should not be viewed as a replacement for proper feeding, stocking density control, water exchange, and pond-bottom management. Instead, it should be considered as one part of an integrated aquaculture water quality management strategy.
7. Water Transparency
Water transparency indicates how deeply light can penetrate into the water column.
For many pond aquaculture systems, a transparency of approximately 25–35 cm is commonly regarded as a suitable range.
Within this range, plankton can be relatively abundant, supporting the growth of filter-feeding fish such as silver carp and bighead carp.
Transparency above approximately 40 cm is generally associated with relatively nutrient-poor or “thin” water.
Transparency below approximately 20 cm may indicate excessively fertile or turbid water.
For many fish ponds, around 30 cm is often used as a practical reference point.
For aquaculture water bodies, transparency can provide a rough indication of plankton abundance and the fertility level of the water.
Experienced fish farmers often use water transparency as a practical indicator when evaluating pond conditions and deciding whether water quality management is needed.
Therefore, water transparency is a useful water quality parameter in pond aquaculture.
7.1 How to Measure Water Transparency
A 25-cm-diameter black-and-white Secchi disk can be used to measure water transparency.
The disk is slowly lowered from the water surface while observing it.
The depth at which the disk can no longer be seen is recorded as the water transparency depth.
In practical farming, experienced farmers may also use a simple visual method.
By extending the arm into the water and observing the palm and fingers, they can roughly estimate water fertility.
If the palm and fingers remain visible when the water reaches approximately the elbow, the water may be relatively nutrient-poor.
If the hand becomes invisible before reaching the elbow, the water may be considered relatively fertile.
Although this method is not highly accurate, it can provide a rough field assessment.
7.2 Factors Affecting Water Transparency
Water transparency is mainly affected by turbidity.
Turbidity refers to the degree to which water becomes cloudy because of fine particles, plankton, and other suspended materials.
During summer, large quantities of plankton can reproduce rapidly, reducing transparency.
During winter, falling temperatures can cause many plankton organisms to decline or settle, increasing water transparency.
Pond bottom conditions can also affect transparency.
Shallow water with large amounts of soft sediment tends to become more turbid and have lower transparency.
A harder pond bottom containing shells, gravel, or other coarse materials may produce clearer water and higher transparency.
In addition, wind, rainfall, and water flow velocity can all affect turbidity and transparency.
In shallow lakes, reservoirs, and slow-flowing small rivers, sediment concentrations may be relatively low, meaning that transparency is often strongly influenced by plankton density.
7.3 Compensation Depth
Light intensity decreases rapidly as water depth increases.
As a result, photosynthesis and oxygen production by phytoplankton gradually decrease with depth.
At a certain depth, the amount of oxygen produced through photosynthesis is approximately equal to the amount of oxygen consumed through respiration by phytoplankton, bacteria, and other organisms.
This depth is called the compensation depth, while the corresponding light intensity is called the compensation point.
The compensation depth helps establish the vertical structure of dissolved oxygen within an aquaculture pond.
The water layer above the compensation depth can be considered the oxygen-producing layer.
As the water becomes shallower, the net oxygen production from photosynthesis generally increases.
The water layer below the compensation depth can be considered the oxygen-consuming layer.
As depth increases, respiration and decomposition can consume increasing amounts of oxygen.
The compensation depth varies considerably among different aquatic environments and aquaculture systems.
Generally, the higher the organic matter content of the water, the shallower the compensation depth tends to be.
The compensation depth of oceans, reservoirs, and lakes is generally deeper than that of ponds.
Intensive fish ponds often have a relatively shallow compensation depth because of their high biological productivity and organic loading.
The concept of compensation depth provides a theoretical basis for determining suitable pond depth.
During the main fish-growing season, the maximum compensation depth of intensive fish ponds is generally considered to be no more than approximately 1.2 meters under the conditions described in traditional pond management references.
For northern ponds during winter, the maximum compensation depth under ice may reach approximately 1.52 meters.
8. pH Value — Acidity and Alkalinity
pH is a measure of the acidity or alkalinity of water.
It represents the activity of hydrogen ions and is one of the fundamental indicators used to evaluate water quality.
In aquaculture, pH is an important parameter because it not only indicates the condition of the water itself but also affects biological, chemical, and physical processes occurring within the aquatic environment.
8.1 Daily Changes in pH
Under normal conditions, pH generally begins to increase after sunrise.
It may reach its maximum value in the afternoon, often around 5:30 p.m., before gradually decreasing again.
pH generally reaches its lowest value before sunrise the following morning.
This daily cycle repeats as photosynthesis and respiration alternate.
A daily pH fluctuation of approximately 1–2 units is often used as a practical reference. Larger fluctuations may indicate an unstable or abnormal pond environment.
The daily change in pH is closely associated with photosynthesis and carbon dioxide (CO₂).
During daylight, phytoplankton and aquatic plants consume CO₂ through photosynthesis. This changes the carbonate system of the water and can cause pH to rise.
At night, photosynthesis stops while respiration continues, increasing CO₂ levels and causing pH to fall.
Understanding this daily pH pattern is important for fish farmers.
If pond pH is low and there is no obvious external pollution source, it may indicate conditions such as relatively low hardness, excessive humic substances, elevated carbon dioxide, insufficient dissolved oxygen, weak photosynthetic activity, excessive stocking density, or reduced microbial activity.
In such situations, overall nutrient cycling and ecosystem metabolism may also be relatively slow.
8.2 pH and Fish Diseases
In aquaculture water, pH can directly or indirectly affect fish growth, development, reproduction, physiological condition, and disease risk.
When pH moves outside the suitable range, normal respiration and metabolism can be affected, resulting in reduced growth and abnormal physiological responses.
Both excessively low and excessively high pH can directly harm fish and, under severe conditions, cause mortality.
Even when extreme pH does not immediately kill fish, prolonged exposure can exceed their tolerance limits, disrupt physiological functions, slow growth, and increase susceptibility to disease.
Low pH — Acidic Water
When pH is too low, fish may become less active and grow more slowly.
Acidic water can also increase the risk of certain diseases and may favor the accumulation of hydrogen sulfide under appropriate chemical conditions.
High pH — Alkaline Water
Very high pH can irritate or damage fish tissues and interfere with normal gill function.
More importantly, increasing pH shifts the equilibrium between ionized ammonium and toxic non-ionized ammonia (NH₃) toward the more toxic NH₃ form.
High pH conditions can also contribute to cyanobacterial problems under nutrient-rich conditions.
Fish generally perform well in neutral to mildly alkaline water.
A commonly referenced suitable range is approximately pH 7.5–8.5, while pH 6–9 can still be considered within a broad tolerance range for many cultured fish.
When pH falls below 6 or rises above 9, negative effects may become increasingly likely.
Under strongly acidic conditions, such as pH below approximately 5.5, fish may experience reduced blood oxygen-carrying capacity and physiological stress.
Feeding activity and digestion can decline, growth can slow, and gill tissue may become damaged.
At extremely low pH levels, severe fish mortality may occur.
Unsuitable pH also affects the availability of important nutrients in the pond.
In alkaline water, phosphorus may form poorly soluble calcium phosphate compounds.
In acidic water, phosphorus can form insoluble iron or aluminum phosphate compounds.
Both conditions can reduce the effectiveness of fertilization.
Around pH 8.5, algae may exhibit favorable growth and strong carbon fixation capacity, while the stability of the aquatic system can also be relatively high.
At around pH 9.5, algal growth may become less favorable.
Very low pH conditions may also be associated with large quantities of dead or stressed algae.
8.3 pH Adjustment
When pH Is Too Low
Agricultural lime or quicklime may be applied where appropriate.
Another option is to discharge part of the old pond water and replace it with fresh water.
When pH Is Too High
Possible management measures include:
- Discharge part of the old water and add fresh water.
- Use appropriate acidifying or humic-acid-based products to adjust alkalinity where suitable.
- Aluminum sulfate or alum may be used in certain water treatment situations.
- Other mineral materials may also be used depending on the specific pond chemistry.
- In cases of acute pH elevation, carefully diluted acids such as hydrochloric acid or acetic acid may be used by trained professionals for emergency adjustment.
- Chemical and biological water treatment products can be used to reduce excessive organic matter, improve pond-bottom conditions, restore balanced phytoplankton communities, and help stabilize pH.
Any chemical adjustment of pH should be based on actual water testing and carried out carefully, because rapidly changing pH can itself cause severe stress to fish.
9. Ammonia Nitrogen
Ammonia nitrogen in water exists primarily in two forms:
- Non-ionized ammonia (NH₃)
- Ionized ammonium (NH₄⁺)
Together, these forms are commonly referred to as total ammonia nitrogen (TAN).
The proportion of toxic non-ionized ammonia increases as water temperature and pH increase.
The equilibrium between NH₃ and NH₄⁺ changes continuously according to water chemistry.
Therefore, ammonia nitrogen concentration can also serve as an indicator of the accumulation and decomposition of nitrogen-containing organic matter in an aquaculture system.
9.1 Sources of Ammonia Nitrogen
In natural waters, ammonia nitrogen mainly comes from the microbial decomposition of nitrogen-containing organic matter through ammonification.
In aquaculture ponds, major sources include:
- Feed
- Fertilizers
- Fish excretion
- Uneaten feed
- Feces
- Decomposing organic matter
As feeding, fertilization, and fish biomass increase, ammonia production can also increase.
9.2 Toxic Effects of Non-Ionized Ammonia
Ammonia is one of the first products generated during the decomposition of nitrogen-containing organic matter.
It can also serve as a nutrient for aquatic plants.
However, excessive ammonia nitrogen can contribute to eutrophication and increase oxygen demand during microbial transformation.
Non-ionized ammonia (NH₃) is much more toxic to fish than ionized ammonium (NH₄⁺).
Even relatively low concentrations can cause stress under sensitive conditions, and toxicity increases as pH and temperature increase because a larger proportion of total ammonia exists as NH₃.
Ammonia can interfere with oxygen uptake and transport, damage gill tissues, reduce feeding activity, and weaken fish resistance to disease.
Under low-oxygen conditions, ammonia accumulation can become more severe.
At high concentrations, ammonia can cause acute toxicity and mortality.
Low concentrations of non-ionized ammonia may cause chronic stress and suppress growth.
At higher concentrations, acute poisoning can occur.
Fish and shrimp suffering from severe ammonia toxicity may show strong agitation, excessive mucus production, tissue congestion, gill and fin damage, abnormal swimming near the surface, and severe respiratory distress.
9.3 Ammonia Nitrogen Control in Fish Ponds
For many carp species, maintaining ammonia nitrogen at a low concentration is important for safe culture.
As ammonia levels increase, fish growth and feeding performance may decline, and at sufficiently high concentrations, severe toxicity and mortality can occur.
Therefore, ammonia is one of the water quality factors that can limit stocking density.
Low dissolved oxygen in bottom water and anaerobic decomposition of organic matter can promote ammonia accumulation.
Consequently, maintaining sufficient dissolved oxygen, especially near the pond bottom, is an important measure for preventing ammonia accumulation and improving water quality.
When ammonium-based fertilizers are applied to shallow ponds, application rates should also consider pH and temperature.
Because higher pH increases the proportion of toxic NH₃, excessive fertilizer application can create a significant ammonia toxicity risk.
9.4 Ammonia Management Measures
Potential management measures include:
- Improve dissolved oxygen and microbial nitrification.
- Use appropriate chemical or biological water treatment products to help transform or remove ammonia.
- Increase aeration and improve water circulation.
- Use suitable organic-acid-based conditioners where appropriate to support water quality stabilization and carbon availability.
10. Nitrite
Nitrite is an intermediate product in the conversion of ammonia to nitrate.
Under well-oxygenated conditions, nitrite can be further converted into nitrate by nitrifying microorganisms.
However, when oxygen availability is insufficient, nitrification can become incomplete or disrupted, allowing nitrite to accumulate.
When oxygen is depleted, nitrogen cycling can become unstable, and nitrate may also be reduced back toward nitrite under certain conditions.
10.1 Causes of Elevated Nitrite
Nitrite problems are often observed during spring and autumn when temperature fluctuations are relatively large.
Reduced microbial activity can disrupt normal nitrogen cycling.
Fertilizers, animal waste, dead algae, uneaten feed, and other organic materials may accumulate and decompose under oxygen-limited conditions, contributing to elevated nitrite.
In aquaculture ponds, excessive stocking density and repeated oxygen depletion can also increase the risk of nitrite accumulation.
High organic loading is another important contributing factor.
When total ammonia reaches a peak, nitrite may subsequently increase and reach its own peak several days later as microbial conversion proceeds.
10.2 Toxicity of Nitrite
Nitrite is generally less toxic than non-ionized ammonia, but it can become a serious problem because ammonia can be converted to nitrite relatively quickly.
When nitrite accumulates to sufficiently high levels, it can cause fish toxicity.
Nitrite can enter the bloodstream and convert hemoglobin into methemoglobin, reducing the blood’s ability to transport oxygen.
This creates a condition similar to internal or functional hypoxia.
At elevated concentrations, fish may show reduced feeding, gill damage, respiratory difficulty, abnormal swimming, reduced responsiveness, and eventually mortality.
Therefore, high nitrite is an important water quality concern in intensive aquaculture systems.
10.3 Nitrite Management
Potential measures include:
- Partial or complete water exchange where appropriate.
- Operate aerators, particularly during periods of oxygen stress, to improve oxygen availability and promote nitrogen cycling.
- Use appropriate water treatment products, including oxidizing pond-bottom conditioners and beneficial nitrifying or heterotrophic bacteria.
- Increasing chloride concentration can reduce nitrite toxicity because chloride ions compete with nitrite for uptake across fish gills. In practice, chloride management should be based on actual salinity and water chemistry rather than applying a fixed dose universally.
- Appropriate phosphorus and carbon/nutrient management can support phytoplankton assimilation of nitrogen and help improve nutrient cycling.
- In severe situations, appropriate detoxification products may be considered according to the specific aquaculture system and water test results.
11. Hydrogen Sulfide
Hydrogen sulfide (H₂S) is a highly toxic, water-soluble gas characterized by a rotten-egg odor.
Under oxygen-rich conditions, hydrogen sulfide is relatively unstable and can be converted through chemical and microbial processes into less toxic sulfur compounds such as sulfate.
In the presence of reactive iron in bottom sediment, sulfide can also form relatively insoluble iron sulfide compounds.
Hydrogen sulfide is mainly generated when sulfur-containing organic matter decomposes under oxygen-depleted and anaerobic conditions.
The toxicity of sulfide in water depends strongly on pH, water temperature, and dissolved oxygen.
Higher water temperatures and lower dissolved oxygen generally increase the risk.
At lower pH, a larger proportion of total sulfide exists as the toxic un-ionized hydrogen sulfide form.
11.1 Causes of Hydrogen Sulfide
Hydrogen sulfide can be produced by sulfate-reducing bacteria under anaerobic conditions, as well as by the decomposition of organic matter.
Under oxygen-depleted conditions, hydrogen sulfide can originate through two major pathways.
First, sulfur-containing organic matter can be decomposed by anaerobic microorganisms.
Second, when sulfate is abundant, sulfate-reducing bacteria can convert sulfate into sulfide, which can then exist partly as hydrogen sulfide depending on pH.
Hydrogen sulfide is more likely to accumulate in old ponds with excessive deposits of weeds, uneaten feed, feces, and other organic matter.
In acidic water, lower pH increases the proportion of sulfide present as H₂S and therefore increases toxicity.
Hydrogen sulfide in pond sediments can also react with metal ions to form dark or black metal sulfides, contributing to the characteristic black color of heavily reduced pond sediment.
11.2 Toxic Effects of Hydrogen Sulfide on Aquatic Animals
Hydrogen sulfide is highly toxic to fish and other aquatic organisms.
Its toxicity is associated with interference with cellular respiration and oxygen utilization.
Hydrogen sulfide can also place additional oxygen demand on the aquatic environment as it undergoes oxidation.
Maintaining H₂S at very low concentrations is therefore important, particularly in intensive aquaculture systems and overwintering ponds.
When hydrogen sulfide begins to accumulate, fish may become restless, feeding activity may decline, feed conversion may worsen, and resistance to stress may decrease.
At higher concentrations, severe neurological and respiratory stress may occur.
Hydrogen sulfide can be rapidly absorbed through the gills and mucus membranes.
It can interfere with oxygen utilization and cause tissue damage, severely affecting fish health and potentially resulting in mass mortality.
Affected fish may show symptoms such as darkened or abnormal gills, extended opercula, abnormal swimming, loss of body coloration, and surfacing behavior.
11.3 Measures to Reduce Hydrogen Sulfide Risk
Increase Dissolved Oxygen
Improving dissolved oxygen is one of the most important measures for reducing anaerobic conditions and preventing hydrogen sulfide accumulation.
Improve Pond pH
Because the proportion of toxic H₂S increases under acidic conditions, appropriate liming can shift sulfide toward less toxic ionic forms.
However, care must be taken when ammonia is already high, because increasing pH can increase the proportion of toxic non-ionized ammonia.
Treat Severe Bottom-Water Problems
In severely affected ponds, appropriate aeration treatments and iron-containing materials may be used to promote conversion of sulfide into relatively insoluble iron sulfide.
Prevent Excessive Sulfate Loading
Water containing excessive sulfate should be evaluated carefully before entering the pond.
The use of sulfate-containing fertilizers should also be managed carefully, particularly in ponds with a history of sulfide accumulation.
12. Self-Regulation and Self-Maintenance of Aquaculture Ecosystems
Ecological theory suggests that under natural conditions, ecosystem stability is maintained through a dynamic balance of structure and function.
When external factors disturb this balance, the ecosystem can respond through physical, chemical, and biological processes and gradually move toward a new equilibrium.
This is the principle of ecosystem self-regulation and self-maintenance.
However, every ecosystem has a limit to how much disturbance it can absorb.
When the intensity of external pressure exceeds the system’s ability to regulate itself, the ecosystem may experience functional disruption and structural damage.
Human activities can introduce nutrients, organic matter, chemicals, and other substances into water faster than the ecosystem can naturally process them.
When the input exceeds the self-purification capacity of the water body, water quality deteriorates and the intended use of the water can be affected.
This is one of the fundamental mechanisms behind water pollution and water quality deterioration.
Therefore, successful aquaculture should not focus only on individual parameters such as dissolved oxygen, pH, ammonia, nitrite, or water temperature.
Instead, farmers should understand how these factors interact within the entire aquatic ecosystem.
The goal of aquaculture water management is to maintain a relatively stable ecological balance, allowing biological, chemical, and physical processes to support one another.
From this perspective, maintaining ecosystem balance should be the starting point for aquaculture water quality management.
When farmers understand the relationships between water temperature, light, algae, dissolved oxygen, transparency, pH, ammonia, nitrite, hydrogen sulfide, organic matter, and pond-bottom conditions, they can make better management decisions and reduce the risk of sudden water quality deterioration.
Aeration is one part of this overall management strategy. In intensive ponds, maintaining adequate dissolved oxygen can support fish respiration, aerobic decomposition, microbial nutrient cycling, and more stable pond conditions.
For farms that require mechanical aeration, the appropriate system should be selected according to pond depth, stocking density, oxygen demand, water circulation requirements, and operating conditions. Paddle wheel aerators, impeller aerators, and air jet aerators can each serve different aquaculture applications.
Ultimately, healthy water is the foundation of healthy fish, efficient feeding, stable growth, and successful aquaculture production.
Frequently Asked Questions About Aquaculture Water Quality
What is the most important water quality parameter in aquaculture?
There is no single water quality parameter that determines aquaculture success. However, dissolved oxygen, water temperature, pH, ammonia, nitrite, and hydrogen sulfide are among the most important factors to monitor. Their effects are interconnected, so water quality should be managed as a complete system rather than by looking at one parameter alone.
What dissolved oxygen level is good for fish farming?
For many aquaculture systems, maintaining dissolved oxygen around 5 mg/L or higher is generally considered favorable. Prolonged exposure to low dissolved oxygen can reduce feeding, growth, and disease resistance and may eventually cause fish mortality.
Why does dissolved oxygen decrease at night?
During daylight, algae and aquatic plants produce oxygen through photosynthesis. At night, photosynthesis stops, but fish, microorganisms, algae, and other organisms continue to consume oxygen through respiration. Therefore, dissolved oxygen commonly reaches its lowest level shortly before sunrise.
How does water temperature affect fish growth?
Water temperature directly affects fish metabolism, feeding activity, digestion, growth, reproduction, and disease susceptibility. Each fish species has a suitable temperature range and an optimum growth temperature. Temperatures that are too low or too high can reduce feeding and growth.
What causes ammonia to increase in a fish pond?
Common sources of ammonia include uneaten feed, fish feces, fertilizers, fish excretion, dead algae, and decomposing organic matter. High stocking density and insufficient dissolved oxygen can make ammonia accumulation more severe.
Why is high pH dangerous when ammonia is present?
As pH and temperature increase, a larger proportion of total ammonia nitrogen exists as toxic non-ionized ammonia (NH₃). Therefore, high pH can significantly increase ammonia toxicity even when total ammonia nitrogen remains unchanged.
What causes nitrite to increase in aquaculture ponds?
Nitrite can accumulate when the nitrogen cycle becomes unbalanced, particularly when there is high organic loading, excessive stocking density, insufficient dissolved oxygen, or inadequate nitrifying bacterial activity.
Why does pond water turn black?
Black pond water may indicate excessive organic matter, accumulated sludge, dead algae, and oxygen depletion. Under these conditions, decomposition can consume large amounts of dissolved oxygen and contribute to the accumulation of harmful substances such as ammonia, nitrite, and hydrogen sulfide.
What causes hydrogen sulfide in fish ponds?
Hydrogen sulfide is mainly produced when sulfur-containing organic matter decomposes under anaerobic and oxygen-depleted conditions. Excessive organic matter accumulation in pond sediments can increase the risk.
How can aeration improve fish pond water quality?
Aeration can increase dissolved oxygen and improve water circulation. Better oxygen availability supports fish respiration and aerobic microbial processes and can help reduce the risk of oxygen depletion and anaerobic conditions.
Which aerator is best for aquaculture ponds?
There is no single aerator that is best for every pond.
Paddle wheel aerators, impeller aerators, and air jet aerators have different operating characteristics. Selection should consider pond depth, stocking density, water circulation requirements, oxygen demand, power availability, and the overall farming system.
For more information, you can compare the applications of our Paddle Wheel Aerator,
and Air Jet Aerator products.
