Since tap water is generally sterilized with chlorine gas, chlorine in tap water is a strong oxidant, which produces a very unstable strong oxidizing agent hypochlorous acid in water.
The oxidant has a strong penetrating lethal effect on fish, which causes the mucus secreted by the fish to lose its protective effect on the fish, causing serious imbalance of electrolyte metabolism in the body and suffocating and dying in the state of hypoxia.
Therefore, before raising fish, it is best to let the tap water in the sun for a day or two, which can reduce the chlorine in the tap water and continuously introduce air or oxygen into the water.
