Sudden Koi Deaths: One Bloated, Five Dead in Hours

Mass Mortality After One Fish Shows Dropsy

15 koi (14"), one swollen (dropsy/kidney failure), five dead within hours of owner's return. Fed bread once prior.

Analysis

• Bread did not kill them—tiny amount, days prior.
• Dropsy fish likely had internal bacterial/kidney issue—not directly contagious.
• But: Decomposition of large dead fish releases ammonia, toxins; if filtration overwhelmed, others die fast.
• Check: Oxygen (hot weather?), ammonia, nitrite, chlorine (recent water change?).

Immediate Actions

1. Remove all dead fish immediately.
2. Test water: ammonia, nitrite, pH, O₂, temperature.
3. 50% water change if any parameter off.
4. Add aeration (air stone).
5. Stop feeding.
6. Add salt (0.1%) and MelaFix for survivors.

See Why Did My Fish Die?

Fast multi-fish death = acute water quality crash. Test and change water now.