Sudden Comet Deaths in a Large Pond: Koi Aggression Suspected

Sudden Comet Deaths in a Large Pond: Koi Aggression Suspected

A 10,000-gallon pond system (8,500-gallon main pond) with 50 comets and 4 koi has perfect water tests, clear water, healthy plants. Yet every few days, 1-2 comets are found dead with no visible trauma or disease. The koi appear to "parent" the comets.

Likely Cause: Hidden Koi Aggression

Koi can be aggressive toward smaller fish, especially in confined spaces. Chasing and nipping may not be observed but cause stress, exhaustion, and secondary infections. The "parenting" behavior is likely herding/chasing.

Recommendations

• Observe at dawn/dusk when activity peaks.
• Provide more hiding places: Dense plants, pipes, rock caves.
• Consider separating comets into a different pond or section.
• Ensure adequate space: 10,000 gallons is good, but 4 large koi + 50 comets may still create territorial pressure.

If aggression is confirmed, rehoming the comets or koi may be necessary for long-term peace.