In aquaculture, farmers often face situations where fish reduce feed intake, eat poorly, or even stop feeding completely. This condition, known as fish anorexia or poor appetite, can last for different periods, vary in severity, and directly affect growth and survival. If not addressed quickly, it can slow fish growth, extend the culture cycle, and cause economic losses.
Based on field observations and research, the six main reasons why fish stop eating or eat less include:
- Disease factors
- Water quality issues
- Harmful algae
- Feed-related problems
- Use of irritating chemicals
- Sudden weather changes
Below, we explain each factor in detail and provide practical solutions to restore fish appetite and ensure healthy growth.
1. Disease Factors
When fish suffer from viral infections, bacterial diseases such as septicemia, streptococcus, enteritis, or gill rot, or liver and bile problems, their ability to digest and absorb nutrients decreases significantly. Sick fish are often weak, leading to reduced or even complete refusal of feed.
Parasites are another major threat. For example:
- Anchor worms (Lernaea) attach to the skin, causing visible red spots, discomfort, and loss of appetite.
- Gill parasites such as Trichodina, Dactylogyrus, and Gyrodactylus stimulate gills, increase mucus secretion, and reduce oxygen uptake, which lowers feeding.
- Tapeworms in the intestine may block digestion, preventing fish from eating.
- Enteritis damages intestinal lining, reducing enzyme secretion and digestion efficiency.
How to respond:
- Diagnose sick or dying fish to confirm the cause.
- Treat bacterial infections with disinfectants.
- Use copper sulfate or specialized products against protozoan parasites like Trichodina.
- Apply Trichlorfon or benzimidazole for worms.
- For crustacean parasites like anchor worms, use pyrethroids or organophosphates (e.g., trichlorfon).
- For intestinal tapeworms, use praziquantel or other deworming feed additives.
👉 In short, treat according to the disease, reduce parasite load, and ensure fish regain health to restore appetite.
2. Water Quality Factors
Low Dissolved Oxygen (DO)
The level of dissolved oxygen in pond water directly affects fish feeding, digestion, and growth. When DO is too low, fish lose appetite, feed conversion rates drop, and growth slows.
For example, studies show that when grass carp are cultured at 3 mg/L DO compared to 6 mg/L, their:
- Feed intake decreases by 35.9%
- Feed conversion ratio increases by 1.34 times
- Growth rate decreases by 64.4%
To ensure normal feeding, DO should remain above 5 mg/L during feeding times. This is why most farmers keep aerators running regularly, especially in summer or at night.
👉 If you want to stabilize pond oxygen levels and prevent fish from going off feed, consider installing reliable aeration equipment such as:
These devices help maintain oxygen supply, reduce stress, and improve feed efficiency.
Other Causes of Low Oxygen
- Excess Zooplankton
When zooplankton such as rotifers, cladocerans, or copepods become too abundant, they consume oxygen themselves and also reduce phytoplankton, lowering overall oxygen levels. Fish may float at the surface or stop eating altogether.
- Solution: Use Trichlorfon or other approved treatments to reduce zooplankton numbers, then restore pond balance.
- Poor Phytoplankton Growth (Lean Water)
Dissolved oxygen mainly comes from phytoplankton photosynthesis. If the water body is too “lean” (low fertility), oxygen production is insufficient, affecting feeding.
- Solution: Apply fertilizers to promote algae growth and maintain DO stability.
- Weather Conditions
Continuous cloudy or rainy days reduce photosynthesis, while hot, humid weather lowers air pressure, both leading to low DO. This is a critical time to increase aeration and prevent fish losses.
Toxic Compounds: Ammonia, Nitrite, Hydrogen Sulfide, Abnormal pH
When harmful substances accumulate in pond water, fish feeding declines:
- Ammonia & Nitrite damage gills and interfere with oxygen transport in the blood.
- Hydrogen sulfide (H₂S) binds with hemoglobin, causing physiological hypoxia.
- Acidic water causes acidosis, protein damage, and mucus buildup on gills.
- Alkaline water corrodes gill tissues and increases mucus secretion, leading to suffocation.
How to manage:
- Add fresh water and increase aeration.
- Apply water conditioners or detoxification agents.
- Use lime for acidic water, or organic acids to reduce alkalinity.
Turbid or Polluted Water
Excess suspended solids (mud, organic waste, chemicals, oil, heavy metals) reduce oxygen transfer and block gill function, lowering feed intake.
Solution:
- Apply lime or coagulants such as polyaluminum chloride to settle particles.
- Increase water exchange and maintain aeration to restore clarity.
👉 Maintaining good water quality and stable oxygen levels is essential to prevent appetite loss in fish. Effective aeration systems not only improve oxygen supply but also reduce harmful gases and enhance overall pond ecology.
3. Harmful Algae Factors
Not all algae are beneficial for aquaculture. While green algae and cryptophytes usually support good water quality and feeding, some harmful algae can cause serious problems for fish appetite and health.
Types of Harmful Algae
- Euglena and Blue-Green Algae (Cyanobacteria)
- These algae are difficult to digest and may irritate the fish gut, causing enteritis and reduced feeding.
- In nutrient-rich ponds, cyanobacteria often overgrow during mid-to-late culture stages. Fish in such ponds usually feed poorly, and the water body becomes unstable.
- Sudden Algal Death
- When large populations of euglena or blue-green algae die during cloudy/rainy weather, they consume massive amounts of oxygen while decomposing.
- This can cause oxygen crashes and even fish kills.
- Toxic Algae (e.g., Microcystis, Prymnesium parvum)
- Some algae release toxins that directly poison fish, damaging the liver, nervous system, or blood cells.
- Fish exposed to algal toxins may show abnormal swimming, weak activity, and sharp reductions in feed intake.
Example: Golden Algae (Prymnesium parvum)
When golden algae bloom, they produce fish toxins, cytotoxins, hemotoxins, and neurotoxins. Once toxin levels rise, fish may suffer mass mortality.
Management practices include:
- Adding fresh water to dilute algal toxins.
- Applying organic acids or ammonium-based treatments to break down harmful algae.
- Fertilizing to promote growth of beneficial algae, which suppresses harmful species.
- After using algaecides, run aerators continuously to prevent oxygen depletion caused by algal die-off.
👉 If you are concerned about sudden algae blooms in your ponds, installing efficient aeration equipment such as Wave Maker Aerator will help maintain water circulation, prevent stratification, and reduce the risk of toxic algae outbreaks.
4. Feed-Related Factors
Proper feed selection and feeding practices are critical to maintain healthy fish appetite. Poor feed quality, inappropriate size, or sudden changes in feed can cause feed refusal or anorexia.
Common Feed-Related Issues
- Hard Feed or Long Soaking Time
- Hard pellets are difficult to digest, may irritate the gut, and can trigger enteritis, eventually causing fish to eat less.
- In early spring, hard feed may sink too quickly, making it difficult for fish to consume, reducing feeding activity.
- Feed Spoilage
- Improper storage or prolonged feed storage can cause feed deterioration or mold, which may lead to toxicity, anorexia, or mortality.
- Always ensure feed is fresh and stored in dry, ventilated conditions.
- Poor Palatability
- Low-quality raw materials or strong-smelling additives reduce feed attractiveness, causing fish to eat less.
- Use high-quality, fresh ingredients to maintain feed appeal.
- Feed Pellet Size
- Too large or too small pellets affect fish feeding efficiency.
- Adjust pellet size according to fish species and body size to improve intake.
- Sudden Feed Changes
- Abrupt feed changes can confuse fish taste perception, reducing feed intake temporarily.
- Gradually mix old and new feed in incremental ratios over several days until fish adapt.
- Unacclimated Fingerlings
- Newly stocked fish that are not properly acclimated may feed poorly initially.
- Perform routine feed acclimation before increasing feeding volume to avoid wastage and growth delay.
By addressing feed-related factors, farmers can improve feeding activity, reduce waste, and accelerate growth. Combined with proper water quality management and oxygen supply through Paddle Wheel Aerators, fish health and appetite can recover faster.
5. Use of Irritating Chemicals
During aquaculture, farmers often apply chemicals to control diseases or pests. Some of these chemicals can temporarily reduce fish appetite due to their irritant effects.
Common Chemicals Affecting Feeding
- Trichlorfon (Dichlorvos), copper sulfate, pyrethroids, and organophosphates are widely used for parasite or pest control.
- While effective against pests, these chemicals can stress fish, causing reduced feed intake for 2–3 days.
Management Tips
- Feed Before Chemical Application
- Always feed fish before using chemicals to minimize appetite loss.
- Use Detoxification Agents After Treatment
- If fish continue to eat less, apply detoxifying agents such as sodium thiosulfate at a rate of 2 kg per 1,000 m³ water to reduce chemical residues and help fish recover.
- Aeration Support
- Running aerators after chemical treatment improves oxygen levels and reduces fish stress.
- Recommended options: Impeller Aerator or Wave Maker Aerator.
By carefully timing chemical application and supporting pond oxygenation, farmers can minimize appetite reduction and maintain healthy feeding.
6. Sudden Weather Changes
Rapid changes in weather can significantly affect water quality and fish appetite. Fish, especially fingerlings, are very sensitive to sudden environmental changes.
Key Weather-Related Factors
- Severe Weather Events (Typhoons, Heavy Rain)
- Sudden rainstorms or strong winds cause fluctuations in water parameters, leading to stress and temporary feed refusal.
- Countermeasures: Monitor weather forecasts, increase aeration, and use anti-stress agents if necessary.
- Cold Fronts (Spring and Late Autumn)
- Frequent cold air movements cause sharp drops in surface water temperature, creating vertical density flows.
- Bottom-layer anaerobic water may rise, releasing toxic substances (nitrite, hydrogen sulfide, and other by-products of organic decomposition) that reduce fish feeding.
- Aerators can help mix water layers, increase oxygen, and mitigate stress.
- Recommended: Paddle Wheel Aerator or Roots Blower.
- Daily DO Variation
- Fish often eat poorly in the morning due to low dissolved oxygen, but appetite improves in the afternoon when sunlight boosts photosynthesis.
- Overfeeding in the evening can cause floating behavior due to high oxygen demand during digestion, leaving undigested feed in the pond and worsening water quality.
Additional Situations Affecting Feeding
Other factors that can cause temporary feed refusal include:
- Chemical treatment or pond medication
- Post-harvest stress
- Breeding season
Practical Recommendations
- Observe and identify the specific cause of appetite loss.
- Adjust feeding strategies: in cooler seasons, feed more frequently with smaller amounts if fish eat little; in warm seasons, feed normally to ensure growth.
- Maintain stable oxygen levels using reliable aeration equipment:
Conclusion
Fish anorexia or poor appetite in aquaculture is usually caused by multiple interacting factors: disease, water quality, harmful algae, feed problems, chemical stress, and sudden weather changes.
By carefully monitoring pond conditions, managing feed, controlling parasites, and maintaining optimal oxygen levels with aeration equipment, farmers can quickly restore appetite, promote growth, and reduce economic losses.
Proper aeration not only improves oxygen supply, but also enhances water circulation, prevents harmful stratification, and supports the overall health of cultured fish.
