Because there are too many fish and there is not enough oxygen. It is best to make the water in the fish tank reach four fifths. Don't change water frequently, and don't change water once or twice a month. Don't empty the water in the fish tank when changing water. You want to clean the fish tank, but in fact it backfires. Therefore, two-fifths of the raw water should be reserved so that the fish can adapt.

Generally, a small round fish tank is about 6 liters. If there is oxygenation and filtration, you can raise up to 2-3 goldfish of 5-8 cm. Of course, it is more comfortable to keep fewer goldfish! It is also easy to gain weight, and the water quality will not change so much! After all, the water quality is not good quickly! If there is no oxygenation and filtration at home, just halve the number above! Of course, I suggest you keep as few as possible! Don't keep any plants and aquatic plants in the fish tank.