Use of Formalin to Control Fish Parasites
Ruth Francis-Floyd  

Formalin is a generic term which describes a solution of 37% formaldehyde gas dissolved in water. Solutions of formalin for use on fish should contain 10 to 15% methanol, which inhibits formation of paraformaldehyde (discussed below), a highly toxic compound. Two commercial products have been approved for use in aquaculture by the Food and Drug Administration (FDA). These are Formalin-F sold by Natchez Animal Supply, Natchez, Miss. and Paracide-F, sold by Argent Chemical Laboratories, Redmond, Wash. Both of these products have been approved by FDA for use on food fish (trout, salmon, catfish, largemouth bass and bluegill) as a parasiticide. There is no legal withdrawal time (time after the chemical was used before fish can be slaughtered for food) for either of these products.

Formalin is used as a bath treatment to control external parasitic infections of fish. It is extremely effective against most protozoans, as well as some of the larger parasites such as monogenetic trematodes. Formalin effectively kills parasites on gills, skin, and fins. It is not the preferred treatment for external bacterial or fungal infections. In addition, high concentrations of formalin are used to control fungi on fish eggs. Formalin is not effective against internal infections of any type.

  • Concerns for safety of personnel
    Formaldehyde is a known carcinogen. It should only be handled by personnel wearing protective clothing such as gloves.

    Formaldehyde is a noxious gas. Formalin must be kept in a sealed container in a well-ventilated area. Exposure to fumes will result in irritation to eyes and respiratory surfaces.

    Some people develop a sensitivity to formalin over a period of time which involves repeated handling of the chemical. These individuals should avoid handling the chemical.

  • Concerns for safety of fish
    Formalin chemically removes oxygen from the aquatic environment. Each 5 mg/l of formalin applied removes 1 mg/l of dissolved oxygen. This is one reason why use of formalin in ponds is discouraged.

    Formalin is an algicide. When applied to pond water, formalin kills a portion of the algae present, thereby reducing the ability of the algae to produce oxygen through photosynthesis. Further decreases in oxygen in the pond can result as the dead algae decompose.

    Formalin should be stored in an area where it is protected from extremes of heat and cold. Never use formalin when storage temperatures fall below 40F (5C) or when a white precipitate (powder) is present. At cold temperatures formaldehyde is transformed into paraformaldehyde (white precipitate), a highly toxic material which will kill fish on contact.

    Formalin toxicity is increased at high water temperatures. If water temperatures exceed 70F (21C), the concentration used should be decreased.

    When treating parasites on sensitive species, such as hybrid striped bass, the concentration of formalin delivered in a prolonged bath should not exceed 10 mg/l.

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