Koi Dying After Heavy Rain: KHV, Oxygen, or Toxin? Diagnostic Steps

Key Clues

• New small koi died first (quarantined, then pond).
• Large koi died later, no visible signs.
• Goldfish unaffected.
• Water tests "normal" but rain was heavy.
• Deaths by size/age order.

Top Suspects

1. KHV (Koi Herpesvirus): Temperature-dependent (70–80°F). Kills koi selectively; goldfish resistant. Causes gill necrosis, sunken eyes, mucus. **Test via PCR (vet/lab).**
2. Oxygen crash from turnover: Rain mixes anoxic bottom water. Large fish die first (higher O2 demand). Goldfish tolerate lower O2.
3. Toxin runoff: Pesticides, hydrocarbons from roads. Selective toxicity possible.

Action Plan

1. Submit a fresh dead koi for necropsy/PCR to a fish vet or state lab. This is the ONLY way to confirm KHV.
2. If KHV confirmed: Depopulate, disinfect (chlorine, lime, dry 30 days), restart. Do not restock koi.
3. If not KHV: Install destratification aeration (bottom diffuser) to prevent turnover kills. Test for toxins.

Survivor Management

Move the last koi and goldfish to a clean, isolated system if possible. Quarantine 30 days. Disinfect equipment.

Don't guess—test. KHV is devastating but manageable *if identified*.