That's disgusting. Fish should be kept well. First, water should not be changed frequently. Second, salt should be added. Third, fish medicine should be put to prevent illness.

In this case, you can only disinfect the fish with potassium permanganate, and the fish will be disinfected with the fish tank for two minutes. Just one or two drugs will do. Add some salt and go to the fish shop to buy medicine for rotten fish.