Koi Dying One by One in a New 300‑Gal Indoor Pond: Overcrowding and Aggression?

Red Flags

300‑gal indoor pond, 1 month old. Koi dying sequentially; dead fish removal triggers next death. Ghost koi shows red patches (internal bleeding?). Stress coat used.

Critical Questions for Diagnosis

• How many koi? What size?
• Fighting observed? (Overcrowding → nipping).
• Water parameters (ammonia, nitrite, pH)?
• Temperature, lighting, plants, other inhabitants?
• Feeding regimen?

Likely Issues

• New tank syndrome – Ammonia/nitrite spikes in uncycled system.
• Overstocking – 300 gal is tiny for koi.
• Aggression – Stressed koi attack weak individuals.

Immediate Actions

Test water daily, do 30% water changes with dechlorinator, add beneficial bacteria, reduce feeding, consider rehoming most koi.