Due to some reasons, the oxygen content in the water body is low, so that the fish died in large numbers due to lack of oxygen. When the oxygenator in the aquarium is turned off at night, the fish must float to the surface to breathe in the early morning of the next day, which is commonly known as floating head. If the shutdown time is too long, the oxygen content in the water is lower than the oxygen consumption of the fish, and the skin of the fish's jaw stretches, and there are congested microvessels on it. If the oxygen content continues to decrease, it will be more difficult for the fish to breathe. At this time, the fish will be irritable, jumping around and dying, and finally die.

For hypoxia, prevention should be given priority to, an oxygenator (pump) should be provided, and water should be changed in a timely manner. Otherwise, because the carbon dioxide exhaust gas released by the fish during the normal metabolic process is soluble in water, when the amount of carbon dioxide dissolved in the water is too high, the fish will become sick.