Sudden Koi Die-Off in Large UK Pond: pH Crash Suspected

11,000-liter fiberglass pond with sophisticated filtration. Three weeks ago, largest koi died; pH slightly off. Neighbor kids threw stones. Then 3 more died; water tested "too acidic." 20% water change + pH buffer helped briefly, but all but one koi died. No visible parasites, fungus, or emaciation.

Likely Causes

• Acute pH crash: Rapid drop stresses/kills koi. Stones may have leached contaminants.
• Toxin introduction: Stones, runoff, or neighbor malice (bleach, pesticide).
• Gill damage: Check gill filaments—should be bright red; pale = toxin/ammonia.

Immediate Steps

1. Test tap and pond water for full chemistry (including heavy metals, pesticides).
2. Large water changes with dechlorinator.
3. Add activated carbon to filter.
4. Contact koi vet or lab (e.g., Yorkshire Koi Forum).

Sudden death of large, healthy koi points to acute water quality disaster.