Three Large Koi Die Suddenly in 1000-Gal Pond: Oxygen or Toxin?

Large Koi Die, Small Fish Fine — Why?

Larger fish have higher O2 demand. If water tests "fine" but big koi die, suspect:

• Low oxygen (heat, rain, plant respiration at night).
• Sublethal toxin (copper, pesticide, ammonia spike) — big fish succumb first.
• Filter overload — biofilter can't handle large fish waste.

Immediate Steps

1. Test O2, pH, hardness, alkalinity, ammonia, nitrite, nitrate.
2. 10% water change + dechlorinator + 0.05% salt.
3. Add air stone (critical).
4. Activated carbon in filter.

Stocking Check

1000 gal = 1 large koi comfortably. You have 3 large + 15 small = overstocked.

Expert Help

Koi club (scrapes, scopes), KoiVet.com, or mail fish for necropsy.