Koi Dying After Heavy Rain and Tap Water Top-Offs: Copper Toxicity?

Rain + Tap Water = Dangerous Combo

Heavy rain causes turnover (mixing low-oxygen bottom water), dilutes buffers, and can introduce pollutants. Topping off with untreated tap water adds chlorine/chloramine and possibly **copper** from pipes. Copper is lethal to koi at 0.15 ppm.

Immediate Steps

1. Stop adding tap water without dechlorinator + heavy metal remover (e.g., Seachem Prime, API Pond Detoxifier).
2. Add a fountain/aerator to maximize oxygen and off-gas volatiles.
3. Run activated carbon in high flow to adsorb copper/organics.
4. Test for copper (special test kit) and ammonia/nitrite.
5. Partial water changes (20% daily) with treated water.
6. Salt 0.1% to reduce stress (if no plants).

Why Goldfish Survive

Goldfish are more tolerant of copper and low oxygen than koi. They're the "canary" that didn't die—yet.

Long-Term Fix

• Install an auto-fill with carbon/RO pre-filter.
• Add a rain diverter or overflow to prevent turnover mixing.
• Keep a copper test kit on hand.

Act fast. The two remaining koi are in critical condition.