New Pond Fish Die-Off: Ammonia Spike from Overstocking

A 250-gallon pond started with 3 fish, then 13 more added at once. Within 7 days, 4–5 die, swimming sideways before death. No visible spots.

Diagnosis: Acute Ammonia Toxicity

Adding 13 fish to a new, uncycled 250-gallon system overwhelms the nascent biofilter. Ammonia spikes cause neurological symptoms (side swimming, lethargy) and rapid death.

Emergency Steps

1. Test ammonia and pH immediately.
2. 50% water change if ammonia >0.5 ppm or pH <7.0.
3. Add bottled bacteria (e.g., FritzZyme, Microbe-Lift).
4. Stop feeding for 3–5 days.
5. Add air stone for maximum oxygen.

Future Stocking Rules

• Add 2–3 fish at a time, max once per month.
• Follow the “1 inch of fish per 10 gallons” rule (conservative for new ponds).
• Cycle the pond with ammonia source before adding fish.

Salt or random medications won’t fix ammonia. Water changes and biofilter maturation are the only cures.