1. Rotten tails and fins in koi fish are generally caused by poor water quality. It is necessary to change water in time to improve water quality, and control feeding at the same time to avoid residual bait in water, otherwise it will pollute water quality.

2. In addition, the water quality should be controlled at ordinary times, and the change of pH and hardness will also stimulate the fish and lead to rot.

3. If you are infected with bacteria, you should be treated with bactericidal drugs.