Detailed Practical Techniques for Water Quality Regulation in Aquaculture

 Water is the living environment for aquaculture animals. Water quality directly affects the growth and development of fish, shrimp, crabs and other aquatic animals, and thus relates to the yield and economic benefits of aquaculture. Therefore, regulating water quality during the production process is very important. In our aquaculture production, we must regulate water quality according to mature, quantified indicators, and maximize the satisfaction of the different physicochemical factor needs of the cultured aquatic animals in the water body, so as to promote their growth and development and achieve the ultimate goal of our aquaculture production.
In the process of aquaculture production, the main physicochemical factors in the water environment that affect the growth and development of cultured organisms include dissolved oxygen, pH, ammonia, nitrite, hydrogen sulfide, etc., and these factors are constantly changing with changes in water temperature, feed application, and the metabolism and photosynthesis of aquatic animals and plants. To obtain good benefits, these physicochemical factors must be well controlled.
1. Dissolved oxygen
Aquatic animals must survive under aerobic conditions; lack of oxygen can cause them to surface for air and die. Therefore, dissolved oxygen is one of the life elements of aquatic animals.
1.1 Dissolved oxygen requirements of aquaculture water:
Generally speaking, the dissolved oxygen in aquaculture water should be maintained at 5-8 mg/L, and at least above 4 mg/L. If dissolved oxygen is low, at best it slows growth and makes disease likely, and at worst it causes surfacing and death; while excessively high dissolved oxygen can cause gas bubble disease in fish.
1.2 Causes of insufficient dissolved oxygen
① Temperature: the solubility of oxygen in water decreases as temperature rises. In addition, the fact that aquatic animals and other organisms consume more oxygen at high temperatures is also an important reason.
② Stocking density: the greater the stocking density in the aquaculture pond, the greater the respiration of organisms, and the greater the biological oxygen consumption, so the pond is prone to oxygen deficiency.
③ Decomposition of organic matter consumes oxygen. The more organic matter in the pond, the more active the bacteria, and this process usually requires consuming a large amount of oxygen to proceed, so it easily causes oxygen deficiency in the pond.
④ Oxidation of inorganic substances: when low-oxidation-state inorganic substances are present in the water, oxidation occurs and consumes a large amount of dissolved oxygen. This causes the dissolved oxygen content in the pond to decrease.
1.3 Reactions of fish, shrimp and crabs during oxygen deficiency
During mild oxygen deficiency, fish and shrimp become restless and agitated; fish surface for air, shrimp move toward the edges, and crabs climb onto the shore; breathing speeds up and feeding decreases or stops. During severe oxygen deficiency, it can lead to the death of fish, shrimp and crabs, causing losses. If the pond water remains in a state of insufficient dissolved oxygen for a long time, the growth of the cultured aquatic animals will stop.
1.4 Relationship between dissolved oxygen and other toxic substances
Maintaining sufficient dissolved oxygen in the water can inhibit chemical reactions that produce toxic substances and convert and reduce the content of toxic substances (such as ammonia, nitrite and sulfide). For example, after organic matter in water decomposes, ammonia and hydrogen sulfide are produced; under conditions where sufficient oxygen exists, through the ammonia-oxidizing decomposition action of microorganisms, ammonia is converted into nitrite and then into nitrate, while hydrogen sulfide is converted into sulfate, producing non-toxic final products. Therefore, maintaining sufficient dissolved oxygen in aquaculture water is very important for aquaculture. If oxygen is deficient, these toxic substances can very easily rapidly reach harmful levels.
1.5 How to increase oxygen
The best and most convenient method is to add fresh water. Where conditions permit, an aerator can be used to increase oxygen. Where conditions do not permit or in emergency situations, an oxygen-increasing agent can be used. Using an oxygen-increasing agent can increase oxygen in the bottom layer of the water body and can also help purify water quality.
 
 
2. pH
pH is an important indicator for measuring water quality. This is because fish, shrimp and crabs each have their own pH adaptation range, and pH determines many chemical and biological processes in the water body. For example, toxic substances such as ammonia and hydrogen sulfide have different forms depending on pH, and their degree of toxicity also differs. Therefore, pH is the barometer of water quality.
2.1 Reference for water pH control
Photosynthesis and respiration of organisms in water and various chemical changes can all cause changes in pH, and its changes have a great impact on fish, shrimp, crabs and water quality. In freshwater aquaculture, pH should generally be maintained between 7-9.
2.2 Direct effects of pH on aquatic animals
Excessively high or excessively low pH is directly harmful to aquatic animals and can even cause death. Acidic water (pH<6.5）可使其血液的pH值下降，削弱其载氧能力，造成生理缺氧症，尽管水中不缺氧但仍使其表现缺氧症状，同时由于耗氧降低，代谢急剧下降，尽管食物丰富，仍处于饥饿状态。pH值过高的水则腐蚀鳃组织，引起鱼虾蟹大批死亡。pH值在低于4或高于10.6时，鱼虾蟹都不能存活。
2.3 Effects of pH on water quality
Excessively high or excessively low pH can both inhibit the activity of microorganisms in water, making organic matter difficult to decompose. When pH is higher than 8, a large amount of NH4+ will be converted into toxic ammonia. When pH is lower than 6, more than 60% of sulfides in water exist in the form of hydrogen sulfide, increasing the toxicity of sulfides. In short, excessively high or excessively low pH will both increase the toxicity of toxic substances in water.