One Dead Fish in 150-Gal Pond: First Warning or Natural Loss?

150-gal pond, koi/shubunkin/comets. Chem tests OK. Good filter + spitter. One dead fish. Water problem or old age?

Single Death = Investigate, Don't Panic

Could be old age, congenital defect, or the first sign of a problem. "Tests OK" with strips ≠ reliable. Liquid test kit for ammonia/nitrite is essential.

Do This

1. Liquid Test Ammonia/Nitrite: Now. If >0 → water changes.
2. Examine Dead Fish: Gills (red? pale? mucus?), body (ulcers, fungus, pineconing), vent (prolapse, worms).
3. Observe Survivors: Gasping? Flashing? Clamped fins? Not eating?
4. Check Filter: Flow? Media clean? Bio-media established?

150 gal is small for koi. They outgrow it fast. Plan upgrade.