Koi Hanging at Waterfall in Heat: Oxygen Crisis

Large koi and tench cluster under waterfall, not eating. Heatwave. Problem?

Yes. Classic Hypoxia.

Warm water holds less O2. Fish metabolism increases → more O2 demand. Biofilter bacteria also consume more O2. Surface gasfall = desperate search for oxygen.

Immediate Triage

1. Test Ammonia/Nitrite: If >0, 33% water change daily + dechlorinator. Stop feeding.
2. Add Aeration: Air pump + large stones at 2–3 ft depth. NOT just surface fountain.
3. Run Waterfall 24/7: Maximize gas exchange.
4. Shade Pond: Shade cloth 50% coverage. Lowers temp 5–10°F.
5. Stop Feeding: Until temps drop and fish resume normal behavior.

Why This Happens

Summer heat + high stocking + inadequate aeration = annual crisis. Plan ahead: size aeration for peak summer load, not average.