Goldfish and Koi Attacking Each Other in Overcrowded Winter Tank

Ten koi, ten goldfish, and three catfish in a 100-gallon indoor tank led to scale eating and flesh exposure on two prime koi. Robyn identifies severe overcrowding as the root cause. Even docile fish attack injured tank mates instinctively.

Immediate Steps

• Test pH, ammonia, nitrite; do 30% water changes several times weekly.
• Separate injured fish into another container (plastic tubs work).
• Feed twice daily, only what's consumed in 3 minutes.
• Lower water temperature to reduce metabolism and aggression.
• Identify catfish species: plecos suck on goldfish/koi; channel cats eat small fish.

Long-Term Solution

Move fish back to the 2-foot deep pond with proper winterization (see guide). Plecos must come indoors. A 100-gallon tank cannot support 23 fish long-term.