Sudden Fish Death in New 90-Gallon Pond: Paint Fumes or Ammonia?

All 9 fish dead in one day in a new pond near an open window during painting? Two prime suspects: paint fumes (VOCs) or new-pond ammonia spike.

Scenario Analysis

Factor	Paint Fumes	Ammonia Spike
Timing	Sudden, all at once	Usually over days
Symptoms	Gasping, erratic swimming, death	Gasping, lethargy, red gills
Water test	Normal params	High ammonia/nitrite
Window proximity	3-4 ft = high VOC exposure	Irrelevant

Immediate Actions

1. 100% water change with dechlorinator.
2. Add activated carbon to filter (removes VOCs).
3. Test ammonia, nitrite, pH.
4. Ventilate area—keep windows closed during painting.

Restarting Safely

• Cycle the pond first (add ammonia source, wait for nitrite→nitrate).
• Stock slowly: 1–2 feeder goldfish ($0.50 each) as testers.
• Wait 2 weeks with zero losses before adding koi.
• Provide shade (floating plants) in sunny spot.

Stocking Rule

Max 1 inch fish per square foot of surface area. 90 gal ≈ 12 sq ft → 12 inches total fish. Your 6-inch koi + 3-inch goldfish = already overstocked.

Don't give up—just cycle properly, stock lightly, and isolate from household chemicals.