Aquaculture professionals all know that water quality indicators are very important! So every day we keep a close eye on parameters such as ammonia nitrogen, sub-salts, pH and dissolved oxygen. But apart from these, there is another "invisible big shot" that is often overlooked, and that is - alkalinity
Today, let's delve into the significance of alkalinity in recirculating aquaculture and the serious consequences that may arise from ignoring it.
What exactly is alkalinity? Why is it important?
Alkalinity is the ability of water bodies to neutralize acids, mainly composed of carbonates and bicarbonates, and can be regarded as the "buffer" of water quality. In the circulating water system, processes such as fish respiration, feed metabolism, and microbial decomposition continuously release acidic substances, and alkalinity is the only defense line to neutralize these acids.
Stabilizing pH value: Alkalinity acts as a "buffer" for water bodies, preventing sharp fluctuations in pH (such as a sudden drop in pH due to the accumulation of CO₂ at night).
Promoting nitrification: Nitrifying bacteria require alkalinity to decompose ammonia nitrogen and nitrite. Insufficient alkalinity causes a sharp drop in the efficiency of the nitrification system!
Affecting the health of fish and shrimp: Low alkalinity can lead to osmotic pressure stress, gill tissue damage, and even induce gill rot and bacterial infections.
2. Reasons for the decrease in alkalinity
The reason why alkalinity continuously decreases in recirculating aquaculture is related to various factors during the aquaculture process.
The farmed species themselves: The respiration of farmed species consumes oxygen in the water and releases carbon dioxide at the same time. These carbon dioxide combine with water to form carbonic acid, thereby reducing the alkalinity of the water body.
Feed: The feeding of feed can also have an impact on alkalinity. The protein in the feed will produce acidic substances such as ammonia nitrogen during digestion, which will further reduce the alkalinity of the water body.
Microorganisms: When microorganisms in water decompose organic matter, they also produce acidic metabolic products, thereby exacerbating the decline in alkalinity.
3. What alkalinity is considered qualified? How to measure?
The normal range of alkalinity in aquaculture varies depending on the aquaculture object and the aquaculture environment, as follows:
Freshwater aquaculture: Generally, the total alkalinity range is between 75 and 200mg/L. For freshwater or low-salinity farmed prawns, the total alkalinity should be greater than 75mg/L. For Marine shrimp farming, the total alkalinity requirement is greater than 100mg/L, and for seedling raising, it is greater than 120mg/L.
Testing misunderstandings:
Only measuring pH but not alkalinity → pH may seem normal, but alkalinity might have been exhausted!
Rough estimation with test strips → Large error, it is recommended to use professional instruments for precise measurement.
4. How can we prevent problems before they occur?
Regularly monitor the pH value of the aquaculture water body. By monitoring the changes in pH value, we can promptly understand the pH status of the water body and thereby determine whether the alkalinity is at a normal level. Once it is found that the pH value drops too fast or too low, immediate measures need to be taken to adjust it.
5. What should I do if the alkalinity is too low?
The alkalinity of water can be increased by supplementing alkaline substances. Commonly used alkaline substances include sodium bicarbonate, calcium carbonate, etc. These substances can neutralize the acidic substances in water bodies, thereby increasing the alkalinity.
When supplementing alkaline substances, we need to determine the addition amount based on the actual situation of the water body and the needs of the farmed organisms to avoid excessive addition that may lead to deterioration of water quality.
6. What are the "alkalinity misunderstandings"?
Misconception 1: "The higher the alkalinity, the better" - When the alkalinity exceeds 500mg/L, the corrosion rate of fish gill filaments increases by 40%.
Misconception 2: "Changing water will solve the problem" - Frequent water changes can cause fluctuations in alkalinity and induce stress-induced death.
Misconception 3: "Quicklime is omnipotent" - Excessive use of quicklime can lead to a pH value greater than 9.5, directly scorching the fish.