Pond Turns Black, Smells Chemical, Fish Gasping: Liner Toxicity?

Symptoms

1000-L pond, large goldfish, years fine. Recent: black water, chemical smell, fish gasping. Tests, water changes, filter, aeration – temporary relief, returns in days.

Suspect: Toxic Liner

If liner is roofing EPDM (not fish-safe), it leaches fungicides, plasticizers → black water, chemical odor, fish death. Fish-safe EPDM is certified.

Other Causes

• Hydrogen sulfide (rotten egg): Anaerobic pockets in deep sludge.
• Methane (fecal): Organic overload.
• Dead animal decomposition.

Diagnosis

1. Describe smell exactly (rotten egg, chemical, plastic, dead fish).
2. Test for heavy metals (copper, zinc).
3. Check liner documentation.

Emergency

1. Large daily water changes (50%) with dechlorinated water.
2. Add activated carbon (lots).
3. If liner toxic: must replace with fish-safe liner. No fix.

Prevention

Only use pond-grade liner. Test water regularly. Maintain biofilter.