Fish Die Overnight in New Rental Pond: No Pump, Storm

Rental home, 3×5 ft pond, ~2.5 ft deep, no pump/aerator. Lived 1 month, added water for evaporation. Bad storm → all but 2 fish dead (4" koi, 1" babies dead; 3" goldfish survive). Two survivors in indoor tank. Asks cause and fix.

Perfect Storm of Failures

• No circulation → stratification, bottom anoxia.
• Storm → temperature crash, mixing of anoxic bottom water, debris influx.
• No biofiltration → ammonia buildup.
• Overstocked for volume (~200–300 gal with 14+ fish).

Immediate Fixes for Survivors

1. Pump: ≥500 gph (2× turnover).
2. Test: pH (AM/PM), ammonia.
3. If ammonia >1.5 ppm: 30% water change (dechlorinate).
4. If pH swing >0.25: ½ lb crushed coral in mesh bag (raises KH).
5. Biofilter: Essential for fish.
6. Dechlorinate all tap water.

Alternatives

Convert to plant-only water garden. Or drain/cover if lease allows. Fish keeping = money pit—only if you enjoy it.