Mosquito Fish Die Overnight in New Pond: Troubleshooting

The Scenario

350-gal pond, new liner, new UV bulb, new filters, conditioned 2 weeks. Water tests "ideal." 12 mosquito fish die in 3 hours; second batch dies overnight. Aeration seems adequate.

Likely Causes

1. pH swing: New ponds often have large daily pH fluctuations (photosynthesis by day, respiration by night). Test 4× daily (dawn, midday, dusk, midnight). If pH varies >0.5, stabilize with carbonate hardness (KH) boosters.
2. Temperature shock: Fish bag temperature vs. pond temperature difference >5°F kills quickly. Float bag 1 hour, adding pond water every 15 min.
3. Chloramine: Tap water may contain chloramine (not removed by standard dechlorinators). Use a chloramine-neutralizing conditioner.
4. Ammonia spike: New biofilter not cycled. Even if tests read zero now, a spike can occur after fish addition. Add bottled nitrifying bacteria.

Step-by-Step Fix

1. Test pH, KH, GH, ammonia, nitrite 4×/day for 2 days.
2. If pH swings, add crushed coral or commercial KH buffer.
3. Use a chloramine remover (e.g., Seachem Prime).
4. Add bottled beneficial bacteria (e.g., FritzZyme 7).
5. Acclimate new fish via drip method over 1+ hours.
6. Add pond salt (1 lb/100 gal) to reduce stress.

Test Fish

After stabilizing, add 2–3 cheap feeder goldfish. If they survive 1 week, the pond is ready.