The reasons why golden koi bite fish:

1. The space is small. Golden koi need enough space. If the activity space is too crowded, koi will bite each other. They want to bite each other to compete for a larger living space.

2. The water quality is unclean. If the water is not changed or the fish tank is cleaned for a long time, the water quality in the tank will deteriorate, and bacteria will appear on the golden koi, which is very uncomfortable. They will bite each other's bacteria.

3. Competition for spouse. During the breeding period, golden koi will chase and bite each other as they compete for mates.

4. Insufficient food. When the owner feeds too much food or raises too much golden koi, they don't have enough to eat. They cannot find food in the tank and will bite other fish.

5. Not familiar with it. The newly added golden koi may be unfamiliar with the golden koi in the original tank and may attack each other.