Winter Koi Losses in 15,000-Gal Pond: Aerator Placement and Biofilter Loss

15,000 gal pond, 6.5 ft deep, 100+ koi/comets. Aerator plates moved from bottom to 2.5 ft shelf per dealer advice. 12 fish dead. Leaf buildup, Microbe-Lift used. Asks if plate move was wrong, normal loss rate, feeding restart.

Aerator Placement

Moving diffusers up prevents supercooling the bottom but reduces oxygen at depth where fish hibernate. Best: mid-depth, centered. Cold/hot water mixing is thermodynamics—can’t be prevented.

Critical Error: Pump/Filter Removal

If pump was removed, biofilter bacteria died. Winter fish kills often stem from lost biofiltration, not temperature. Keep pump running through a biofilter all winter.

Action Plan

• Test pH, KH, ammonia, nitrite, nitrate, salt.
• Remove leaves with long net.
• Restart pump/biofilter if off.
• Feed at 50°F+ (wheat germ); summer food at 60°F+.
• Ozone injector option for redox potential (advanced).

Dealer Accountability

High-volume customer deserves on-site support. Print this; discuss with owner.