Leeches swimming underwater in a tank, attaching to the glass. Freshwater leeches (family Hirudinidae) are parasites that have two suckers, one at each end of the body and used to attach to the skin of a victim while drinking their blood. Freshwater leeches are found in lakes and marshes, as well as slow-flowing streams. They feed on the blood of fish, amphibians and mammals. Leeches are also used for medicinal purposes.