Leeches in a Lily Pond Connected to a Koi Pond: Risks and Removal

Small Leeches in the Upper Pond: Threat to Koi?

Finding numerous small leeches in your upper lily pond—especially attaching to you during maintenance—indicates a breeding population. They will circulate through the stream and biofilter into the main koi pond.

Risk to Koi

Leeches feed on fish blood, causing anemia, stress, and secondary infections. Heavy infestations can kill small koi.

Treatment Options

• Potassium permanganate – 2 ppm bath in the upper pond (repeat in 3–4 days). Kills leeches and eggs. Remove plants first or treat in a separate container.
• Commercial leech treatments – products like "Leech-X" or "Anti-Leech" (check aquarium/pond suppliers).
• Physical removal – bait traps (raw liver in a perforated jar) overnight.
• Break the cycle – dry out the upper pond completely for 2 weeks if possible.

Prevention

Quarantine new plants (leech eggs hitchhike). Inspect and rinse thoroughly before adding.

Treat promptly—leeches multiply rapidly in warm, organically rich water.