Formalin Treatment for External Parasites: Uses and Serious Cautions

Formalin (formaldehyde solution) is an FDA-approved, highly effective treatment for external protozoan and monogenean parasites and egg fungus - but it depletes oxygen, is a human carcinogen, and can harm sensitive fish and biofilters. Learn its uses and the safety rules that make it work.

What formalin treats

Formalin is a solution of about 37% formaldehyde gas and is FDA-approved as a parasiticide for finfish and to control water mold ('fungus') on fish eggs. It is extremely effective against external parasites on the skin, fins and gills - protozoans such as Ich, Trichodina, Chilodonella, Costia and Epistylis, and monogenetic trematodes such as Gyrodactylus and Dactylogyrus. It does not treat internal infections of any kind.

How it is used

• As a prolonged or indefinite bath at a low concentration (university guidance cites roughly 12.5-25 mg/L, with sensitive species at the lower end), with strong aeration.
• As a short bath at a higher concentration for a limited time.
• On fish eggs at a high concentration for a brief period to control fungus.

Because parasite eggs resist treatment, monogenean control may require a repeat after several weeks. Follow the specific product label and dose to your exact water volume.

Sources: UF/IFAS VM061, Use of Formalin to Control Fish Parasites (ask.ifas.ufl.edu); UF/IFAS FA033, Monogenean Parasites of Fish (ask.ifas.ufl.edu); Merck Veterinary Manual, Management of Aquarium Fish (www.merckvetmanual.com); FDA, Approved Aquaculture Drugs (www.fda.gov).