This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Add a copper remedy to the tank and monitor it with a copper test kit.
Also, Mardel's Maroxy works well. For anchor worms or leeches on pond
fish, remove them from the affected fish with tweezers and swab the
area with mercurochrome to prevent infection.