New Koi Die After Pond Clean: Chlorine, Cycling, or Bad Luck?

6x8x2 ft pond, inherited koi/goldfish. Winter plants covered surface. Spring: removed plants, found few survivors. Scrubbed bottom, 40% water change, cleaned filter, added salt. Added 4 koi + 12 goldfish. Next day: all but 1 koi dead. Water tested “fine” with slight chlorine elevation.

Likely Causes

• New pond syndrome: Scrubbing destroyed biofilter. Ammonia spike killed new fish.
• Chloramine in tap water: “Slight chlorine” may be chloramine; salt doesn’t neutralize it.
• Temperature/pH shock: No acclimation described.
• Overcrowding: 16 new fish in 720 gal = high load for new biofilter.

Correct Restart Protocol

1. Let pond sit 1–2 weeks after cleaning; test ammonia/nitrite daily.
2. When both 0, add 2–3 fish, acclimate 30 min (float bag, slow mix).
3. No salt, no medications unless disease confirmed.
4. Feed sparingly; monitor water daily for 2 weeks.

The “fine” water test missed ammonia/nitrite. A cycled filter is non-negotiable.