1. Malnutrition: Long-term malnutrition may cause koi fish to be big and small. In order to maintain the body shape, the feeding amount may be reduced, which may lead to the deterioration of fish's lower body constitution in the state of long-term hunger, or the invasion, or malnutrition, gastrointestinal dysfunction, and no longer absorb nutrition.

2. Poor water quality: Poor water quality may lead to slow growth and emaciation in koi fish.