Mass Koi/Goldfish Die-Off: Cottonwool Disease, Salt Bath & Overcrowding

25 fish → 3 left (2 goldfish, 1 large koi gasping). Blood on bellies/fins, cotton bodies, cloudy eyes. Cottonwool disease (Columnaris) from overcrowding.

Emergency Treatment

1. Salt bath: 3 kg salt / 100 L water (3% = 30 g/L). 10 min max.
2. Hospital tank: Survivors to clean, aerated, heated (75°F) tank.
3. Pond disinfection: Drain, scrub, dry, UV, refill.
4. Meds in hospital: Kanaplex + Furan-2 (broad-spectrum).

Root Cause: Overcrowding

• 25 fish in unknown volume — likely severe.
• Stress → immune collapse → Columnaris epidemic.
• Melafix/Pimafix = weak, useless for this.

Survivor Reality

• Gasping koi = gill damage — poor prognosis.
• Pink goldfish = septicemia (blood in tissue).
• Injection (Baytril) = vet only, last resort.

This was preventable. Stocking density killed them. Salt bath + hospital tank + disinfection = only shot. Reduce stock 80% if restarting.