Koi and Goldfish Lose Appetite: Aeration, Food Quality, or Water Chemistry?

Fish in a two-tier pond (top: 4' deep, 4'x5'; bottom: 1.5' deep) with 2 koi, 6 goldfish (top) and 1 goldfish (bottom) suddenly ignore food after weeks of voracious feeding. Waterfall aeration runs only 5 hours/day. Fish appear healthy, graze algae constantly.

Three Primary Suspects

1. Insufficient aeration: 5 hours/day is inadequate for biofilter function and fish metabolism, especially at 76°F. Low oxygen suppresses appetite.
2. Poor food quality: “Pond sticks” may lack essential nutrients. Summer staple or high-protein pellets are better. Seasonal diet transition (wheat germ in cold, protein in warm) matters.
3. Water chemistry imbalance: Ammonia, nitrite, pH swings, or low KH can all reduce feeding drive.

Action Plan

• Run waterfall 24/7 or add a dedicated air pump.
• Switch to a high-quality summer koi food.
• Test ammonia, nitrite, pH, KH, and nitrate.
• Observe: if fish eat algae but not pellets, the pellets are the issue.

Address aeration first—it’s the most common and easily fixed cause of appetite loss in warm weather.