Why 6 Koi Died in a 10,000‑Gal, 9‑Ft Deep Pond: Bottom Oxygen Depletion

Setup

New 10,000‑gal, 9‑ft deep liner pond (Wisconsin). 6 adult koi + 14 juveniles (removed). Aerator kept surface hole open. Leaves accumulated. Winter mild; ice 6 inches thick.

Likely Cause

Aerator only at surface → bottom water stagnant. Koi rested at 9‑ft depth where oxygen dropped, CO₂/H₂S/methane rose from decaying leaves. Large koi need more oxygen.

Fix for Next Winter

• Add a **bottom‑diffused aeration system** (air stone at pond floor).
• Example: pond aeration kit.
• Slight disturbance is better than suffocation.

Lesson

Deep ponds need bottom aeration in winter, not just a surface hole.