Dissolved oxygen (DO) in water is the most fundamental condition for the survival of aquatic animals. The DO content directly affects stocking density, yield, and the efficiency of water surface utilization. In addition to oxygen entering water through diffusion and photosynthesis, the primary source of oxygen in aquaculture is mechanical aeration.
Although aerators are widely used, many people still regard them only as “fish-saving machines” rather than “yield-enhancing machines.” This underestimation often leads to oxygen deficiency incidents or even latent chronic low-oxygen conditions, which can trigger fish disease outbreaks. Therefore, this article summarizes the data related to DO, the working principles, and functions of aerators.
I. Characteristics of Oxygen
| Physical Properties of Oxygen | Description |
|---|---|
| Color | Colorless |
| Odor | Odorless |
| State | Gaseous |
| Density (Standard Conditions) | 1.429 g/L |
| Solubility | Slightly soluble in water |
| Boiling Point | -183°C |
| Melting Point | -218°C |
From the table above, we understand that oxygen is a colorless, odorless, and tasteless gas. Oxygen molecules can dissolve in water, but its solubility is very low—around 30 mL of oxygen dissolves in 1 liter of pure water at room temperature and standard atmospheric pressure. Due to this low solubility, artificial or mechanical aeration is necessary to increase oxygen levels in aquaculture environments.
II. Relationship Between Dissolved Oxygen and Atmospheric Pressure
| Air Pressure (kPa) | Dissolved Oxygen in Water (mg/L) |
|---|---|
| 99.6 | 6.0 |
| 100.0 | 6.8 |
| 100.5 | 7.9 |
| 100.9 | 8.7 |
| 101.2 | 8.3 |
| 101.8 | 8.6 |
| 102.3 | 9.3 |
As shown in the table, the higher the air pressure, the higher the DO concentration in water. Lower pressure leads to potential oxygen depletion or stress reactions in fish. Stress in fish refers to non-specific physiological responses to environmental changes, leading to sluggish behavior, reduced appetite, restlessness, or even death. Mechanical aeration should be activated during such events.
III. Impact of DO Levels on Fish Feeding
DO levels not only affect survival but also feed efficiency and water quality. Research shows:
- When DO drops from 4.5 mg/L to 3.5 mg/L, feed digestibility decreases by 50%.
- At 3 mg/L, feeding behavior is affected.
- Below 1 mg/L, fish begin surfacing for air (a sign of oxygen distress).
- Prolonged low DO can cause massive fish deaths (“pond turnover”).
IV. Calculating Oxygen Demand in Aquaculture
- Theoretically, digesting 1 kg of feed requires 1 kg of oxygen.
- 1 kWh of electricity can produce about 3.6 kg of oxygen.
- In natural conditions without aeration, fish-carrying capacity is about 300 kg/acre.
- In extreme weather, 150 kg/acre may still lead to oxygen deficiency.
Example Calculation:
For an expected yield of 1,000 kg/acre with a 3% feeding rate, 21 kg of oxygen is needed daily. This requires about 5.8 kWh of energy. A 1.5 kW aerator should run for 3.8 hours per day.
Emergency Aeration:
An impeller aerator rated at 1 kW can add more than 1.8 kg of oxygen per hour—enough for several tons of fish—effectively preventing oxygen depletion.
V. Effects of DO Levels on Aquatic Animal Health
Advantages of Proper Oxygenation:
- Energy Saving – Bottom-mounted microporous aerators are efficient, simple, and long-lasting. A 2.2 kW unit can effectively aerate 30–40 acres.
- Improved Environment – Microbubbles rise from the pond bottom, enhancing DO levels, oxidizing harmful substances, and promoting water quality stability.
- Convenience – Suitable for dual-pond or multi-pond setups. Easy to install and replace tubing.
- Safe from Electric Leaks – The motor remains onshore; only the pipes go underwater.
Disadvantages:
- Higher Initial Cost
- Risk of Blockage in microporous pipes
- Not Ideal for Deep Ponds (depths over 1.5–2.0 meters reduce effectiveness)
DO Thresholds:
- ≤ 2 mg/L: Water becomes foul; risk of fish suffocation.
- 2–5 mg/L: Fish survive but grow slowly; yield is limited.
- ≥ 5–7 mg/L: Optimal for growth and reproduction.
Low DO leads to reduced movement, appetite, immunity, and reproductive capacity.
VI. Principle of Oxygen Dissolution by Aerators
Aerators do not generate oxygen directly. Instead, they circulate water to enhance contact between water and air, increasing oxygen absorption. Larger contact areas and stronger flow (from higher wattage or larger splash zones) result in faster oxygenation.
During daylight, oxygen levels rise due to photosynthesis. At night, respiration by aquatic organisms and microbes consumes oxygen, making nighttime aeration essential.
VII. Types of Aerators
There are several types of mechanical aerators, including impeller, paddle wheel, fountain, jet, wave-making, air pump, and microporous models. Below are brief introductions to the most common four:
1. Fountain Aerator
Principle: Pumps low-oxygen water from the bottom and sprays it into the air.
Pros:
- Increases surface contact area and oxygen absorption
- Decorative effect for parks and garden ponds
Cons:
- Small effective area
- High energy consumption
- Best suited for small ponds or emergency oxygenation
2. Impeller Aerator
Principle: Circulates and mixes water using impellers; draws air into the water via suction and forms microbubbles.
Pros:
- Effective up to 2 meters deep
- Simple design and low maintenance
- Promotes uniform DO in upper layers
- Removes toxic gases
Cons:
- Fixed location limits aeration coverage
- Floating parts degrade under sunlight
- Noisy, may stress fish
- Not ideal for shallow ponds
Most commonly used in ponds over 1 meter deep due to high oxygenation efficiency.
3. Paddle Wheel Aerator
Principle: A motor-driven vertical paddle churns water to create surface waves.
Pros:
- Lightweight, simple structure, low cost
- Effective in shallow ponds
- Strong upper-layer circulation; good for large-scale operations
Cons:
- Poor performance in deep water
- Not suitable for emergency use during oxygen deficiency
4. Wave-Maker Aerator
Principle: Pumps water upward through a central shaft to create circular waves.
Pros:
- Lightweight, energy-efficient
- Promotes sunlight penetration and algae growth
- Enhances natural oxygenation
Cons:
- Stirs mud in shallow ponds
- Less effective during overcast or rainy weather
Ideal for large or deep ponds, but not recommended for shallow or heavily polluted waters.
5. Microporous Aerator
Principle: Roots blowers supply air to microporous tubes that release microbubbles from the pond bottom.
These bubbles rise slowly, dissolving oxygen efficiently, and drive water circulation to evenly distribute oxygen.
VIII. Principles for Configuring Aerators
Aerator selection must meet the oxygen needs of fish, prevent DO-related diseases, and minimize cost. Configuration depends on pond depth, area, yield goals, aerator efficiency, and power consumption.
- Typical intensive ponds yield 700 kg/acre, requiring 0.3–0.4 kW/acre.
- A 10-acre pond should have two 1.5 kW aerators.
- High-yield ponds (>800 kg/acre) should have 3 kW units for every 2–3 acres.
Impeller aerators offer the highest efficiency—up to 2 kg O₂/kWh—suitable for 4–5 acres per kW. Ponds larger than 5 acres should use multiple units.
In short:
- Lower load (0.8–1.2 acres/kW) = faster and more effective aeration
- Higher load (e.g., 5–10 acres/kW) = slower oxygenation, requiring early activation
IX. Installation Guidelines for Aerators
Installation should consider pond shape, size, and depth:
- Rectangular ponds → Paddle wheel aerators
- Square or circular ponds → Impeller aerators
- Deep ponds → Wave-makers
- Shallow ponds → Floating pumps
Aerators should be installed centrally or upwind, at least 5 meters from pond edges. Use poles, anchors, or ropes for stabilization. Take precautions to avoid stirring sediment, especially with wave-makers. Electrical safety and routine maintenance are essential.
