The rotten tail and fins caused by bacteria are highly contagious, starting with the fins and then rotting to death. This fin rot has two symptoms. The first one starts from the edge of the fin and then extends inward. The second one starts from the central part of the fin and spreads in all directions. The affected area turned white and finally fell off. Goldfish's body resistance will be weakened after transportation or spawning, and they are most vulnerable to this disease at this time. Once a sick fish is found, it must be isolated immediately and treated with antibiotics or yellow. When the symptoms of fin rot gradually appear, the treatment method is to replace part of the tank water, clean the filter and add a little salt to the tank, and stop feeding for a few days. The function of salt therapy is to increase the proportion of water, change the osmotic pressure in water, and kill bacteria and other pathogens through the change of osmotic effect. It can also be directly treated with commercially available Sera germicide, American sterilization, queen whale goldfish disease special agent, queen whale bactericide and AZ00 bactericide.