Fish Dying After Pond Liner Replacement: New Pond Syndrome

10-year pond, moved with fish, heron damage, liner replaced, biofilter added. Told "water too clean," fish dying. Cleaned filter, added feeder goldfish—most died. Survivors bottom-hugging, not eating. Added salt, now hair algae.

What Happened

• New liner = sterile system: No beneficial bacteria, no biofilm (fish slime coat needs bacteria).
• Over-cleaning filter: Removes established bacteria.
• "Too clean" water: Lacks microbial diversity—fish lose protective slime, succumb to stress/disease.
• Heron may have introduced pathogens.

Recovery Plan

1. Add beneficial bacteria (Microbe-Lift, etc.) weekly for month.
2. Stop cleaning filter except gentle rinse in pond water.
3. Add pond salt (0.1%) for slime coat support.
4. Barley straw for hair algae.
5. Feed sparingly high-quality food.
6. Test ammonia/nitrite daily until stable.

Pond needs 6–12 months to mature. Patience + bacteria = recovery.