1. Reason: The blackening of goldfish tail may be caused by poor water quality. When the content of ammonia, nitrogen and nitrate in the water in the fish tank is high, their tail may be congested and blackened.

2. Treatment: The breeder needs to change the water of goldfish immediately, which can reduce the content of harmful substances in the water. If conditions permit, a circulating filter can be installed, which can also effectively clean up impurities and harmful substances in the fish tank.

1. Reason: Goldfish may suffer from enteritis when their tails turn black. If the breeder feeds them with irritating or unclean things, their tails will turn black. In addition to blackening their tails, their bodies also turn black.

2. Treatment: Breeders need to stop feeding them immediately. If their state is not affected, it is most appropriate to feed them with some bait. If their feeding status has been affected, the breeder needs to give them a medicated bath with furazolidone or penicillin.

1. Reason: Goldfish tail blackening may also be due to vertical scale disease. When they suffer from vertical scale disease, the scales of their bodies will stand up and their tails will turn black.

2. Treatment: Breeders need to use yellow powder to give sick fish a medicated bath, and add some crude salt to the water in which they live. Finally, they are fed some high-nutrition food, which can speed up their recovery.