Mass Koi Death After Adding Filter Sponges: What Happened?

A pond owner's grandfather lost every koi in his two-year-old pond within hours of placing new sponges in the filters. The fish were gasping at the surface before dying. The sponges had been added to combat algae, but they may have introduced a deadly toxin.

Likely Cause: Chemical Contamination

The sponges were almost certainly the culprit. If they were previously used for cleaning, industrial purposes, or treated with antimicrobials, they could leach chemicals like formaldehyde, phenols, or heavy metals into the water. Koi gasping at the surface indicates severe oxygen depletion or gill damage from toxins—both consistent with acute poisoning.

What to Do Now

• Remove the sponges immediately and perform a large water change (50% or more) with dechlorinated water.
• Add activated carbon to the filter to adsorb remaining toxins.
• Aerate heavily with air stones or a waterfall to restore oxygen.
• Test water for ammonia, nitrite, pH, and any unusual odors or colors.

Prevention

Only use filter media labeled safe for aquariums/ponds. Rinse new media thoroughly in dechlorinated water before installing. Never repurpose household or industrial sponges.

This tragedy underscores the importance of verifying every product that contacts pond water. Our condolences on the loss.