"Taking care of water is the key to raising fish." This well-known principle in the fish farming industry highlights the supreme importance of water quality management in fish farming. The quality of water directly affects the health, growth rate and final yield of the farmed organisms.
So, what key indicators need to be tested? How can we determine if the water quality meets the standards? Today, we will comprehensively analyze the "water quality code" of aquaculture.
1. Dissolved Oxygen (DO)
Core function
Fish and shrimp obtain oxygen from water through their gills. Insufficient dissolved oxygen can cause fish to surface to breathe (hypoxia) and lead to suffocation and death. It can also cause an increase in ammonia nitrogen and nitrite levels in the water (resulting in the proliferation of anaerobic bacteria).
Safety range
Freshwater fish: 5-8 mg/L (minimum not less than 3 mg/L)
Fish and shellfish: 6-9 mg/L (More sensitive to hypoxia; below 4 mg/L is prone to stress)
Practical operation points
Detection time: Focus on the period from 6 to 8 in the morning (no photosynthesis at night, and the dissolved oxygen level is the lowest)
II. pH Value
Core function
It affects the metabolism and enzyme activity of fish and shrimp, and also alters the forms of toxic substances in the water body (for example, under acidic conditions, the toxicity of ammonia nitrogen decreases, but the toxicity of heavy metals increases).
Safety range
Neutral to slightly alkaline is the most suitable: 7.5 - 8.5 (Beyond the range of 7.0 - 9.0 will cause stress)
Practical operation points
High pH (above 9.0): This is often caused by excessive growth of algae (which can exceed the limit after sunny afternoons). Use organic acid-based regulators to neutralize it, or replace the water in moderation.
Low pH (less than 7.0): This is mostly caused by acidic bottom conditions or excessive leftover food. Sprinkle slaked lime (5-10 kilograms per acre, dissolved and sprayed) to improve the bottom quality.
III. Ammonia Nitrogen (NH3-N)
Core function
The metabolic waste of fish and shrimp, along with the decomposition products of leftover food, the molecular form of ammonia (NH3), is extremely toxic to fish and shrimp. It can damage the gill tissues and affect their breathing.
Safety range
≤0.2mg/L (during the seedling stage ≤0.1mg/L, for shrimp and crabs ≤0.15mg/L)
Practical operation points
Control measures: Reduce excessive feeding and promptly remove leftover food.
Transformation: Use nitrosifying bacterial preparation (convert ammonia nitrogen into nitrite, and then convert it into nitrate)
Emergency: Replace water by 1/3, or sprinkle zeolite powder (20-30 kg per acre, to adsorb ammonia nitrogen)
IV. Nitrite (NO2-N)
Core function
The intermediate products of ammonia nitrogen conversion can damage the oxygen-carrying capacity of fish and shrimp's hemoglobin, leading to "hypoxic poisoning" (even though there is sufficient dissolved oxygen, the fish and shrimp still float to the surface).
Safety range
≤0.1mg/L (during the seedling stage ≤ 0.05mg/L, for South American white shrimp ≤ 0.08mg/L)
Practical operation points
Supplement beneficial bacteria: Bacillus + Nitrogen-fixing bacteria, accelerating the conversion of nitrite into harmless nitrate.
Increase dissolved oxygen: Adequate dissolved oxygen is crucial for the operation of nitrifying bacteria.
Adding salt: A moderate amount of salt can be added to seawater (5-10 kg per acre) to reduce the toxicity of nitrite.
V. Nitrate (NO3-N)
Core function
Non-toxic and highly effective as a nitrogen fertilizer for algae, but excessive use can cause the algae to grow wildly, leading to algal blooms.
Safety range
≤20mg/L (Excessive levels may lead to the outbreak of cyanobacteria and green algae)
Regulatory techniques
Replace the water in moderation (replace 1/4 to 1/3 of the water volume)
Planting aquatic plants (such as duckweed and water hyacinth) to absorb nitrate
Control the use of nitrogen-based fish medication / fertilizer products
VI. Hydrogen Sulfide (H2S)
Core function
Under anaerobic conditions (when the substrate turns black and the sludge is too thick), this substance is produced through the decomposition of leftover food and feces. It is extremely toxic to fish and shrimp, and even at low concentrations, it can cause death.
Safety range
≤0.01mg/L (If the detected value is greater than 0.05mg/L, immediate action is required.)
Practical operation points
Improve the bottom quality: Regularly use bottom conditioner (peroxide calcium, zeolite powder) to oxidize the bottom silt.
Increase the dissolved oxygen at the bottom: Install bottom aeration equipment for oxygenation.
Emergency water replacement: Remove the sewage from the bottom layer and replenish with fresh water.
VII. Transparency
Core function
It reflects the content of algae and suspended matter in the water body. Water that is too turbid or too clear is not conducive to aquaculture.
Safety range
25 - 40 cm (able to see the marker 20 - 30 cm underwater)
Practical operation points
Low transparency (less than 20 cm): The water body is overly rich in nutrients and has a high amount of suspended solids, which can lead to oxygen deficiency. Use flocculants (such as polyaluminum chloride) for sedimentation, or replace the water.
High transparency (greater than 50 cm): The water body is lean, with few algae, and has a weak oxygen-producing capacity. Appropriate application of organic fertilizers (composted chicken manure, pig manure) or biological fertilizers is recommended to cultivate beneficial algae.
VIII. Total Hardness (GH)
Core function
The total content of calcium and magnesium ions affects the bone development of fish and shrimp (for example, shrimp and crabs need calcium when molting), and also stabilizes the pH value.
Safety range
50 - 200 mg/L (calculated based on CaCO3)
Practical operation points
Insufficient hardness (less than 50mg/L): Shrimp and crabs have difficulty molting and have a low survival rate. Sprinkle calcium chloride (3-5kg per acre) or dolomite powder (10-15kg per acre) on the water surface.
Hardness too high (> 300mg/L): It can easily cause water turbidity. Dilute it by replacing some of the water.
IX. Total Alkalinity (TA)
Core function
To assess the buffering capacity of water bodies (resistance to drastic pH fluctuations), when the total alkalinity is insufficient, the pH value is prone to sudden rises and drops, causing severe stress to fish and shrimp.
Safety range
80 - 150 mg/L (calculated based on CaCO3; for shrimp and crab ponds, it is recommended to be ≥ 100 mg/L)
Regulatory techniques
Insufficient alkalinity: Sprinkle sodium bicarbonate (baking soda, 5-8 kg per acre) or slaked lime (in small amounts multiple times)
Alkalinity is too high: Replace the water in moderation, or use organic acid regulators to adjust.