Big Fish Dying After New Additions: Disease or Ammonia Spike?

10-year pond, pump off for years. Restarted pump/fountain. Old fish (8 yrs) hiding. Bought 10 small fish. Now losing 1 big fish every 2 days. Some new ones dead too. Plants taking over. 8x4x2.5 ft (~600 gal). Ohio.

Two Scenarios

1. New Fish Brought Disease: No quarantine = introduced parasites/bacteria. Old fish have no immunity.
2. New Fish Overwhelmed System: 10 new fish + 17 old = 27 fish in 600 gal = massive bioload. Pump/fountain ≠ biofilter. Ammonia/nitrite spiked → toxic.

Test NOW

Ammonia & nitrite. If >0:

• STOP FEEDING.
• 33% daily water changes (dechlorinate, temp match).
• Add bottled bacteria (instant cycle).
• Add air stones.

If 0: likely disease. Treat with broad-spectrum (Prazi + Metro) in quarantine. But test first—ammonia is faster, cheaper, and more common.

Plants Taking Over = Nutrient Sink (Good) But Also Debris Source

Thin them. Remove dead foliage. Install bottom drain or vacuum annually.