Tiny Green Worms in Pond Filter: Insect Larvae, Not Parasites

What You're Seeing

Green worms in filter media are almost certainly **caddisfly larvae** or **midge larvae** that have ingested algae, giving them a green color. They build tiny tube cases from silk and debris. They are harmless—even beneficial—as they process waste.

No Need to Kill Them

• They don't infect fish or humans.
• Fish eat them if they escape the filter.
• They indicate a healthy, biodiverse filter.

If You Must Reduce Numbers

• Rinse media in pond water (not tap) during cleaning—some will wash out.
• Bti (mosquito dunks) kills midge larvae but not caddisflies.
• Don't use pesticides—they'll crash your biofilter.

Leeches (if they attach by suction) are also usually harmless detritivores. Parasitic leeches are rare in ornamental ponds.

Embrace the biodiversity. A sterile filter is a dead filter.