Formalin in Fish Treatment: Uses, Dosing and Safety
How formalin (a ~37% formaldehyde solution) is used against external protozoan and monogenean parasites in fish, with FDA- and UF/IFAS-based dosing and safety notes.
Overview
Formalin is an aqueous solution of formaldehyde (CH2O), the simplest aldehyde. The FDA-approved aquaculture product Parasite-S (NADA 140-989) defines formalin as a solution of about 37% by weight of formaldehyde gas in water, equivalent to roughly 37 grams of formaldehyde per 100 ml of solution. Commercial formalin usually contains a small amount of methanol as a stabilizer to suppress oxidation and polymerization. In fish health, formalin is applied directly to the water as a parasiticide and, on eggs, as a fungicide.
What it treats
According to the FDA Parasite-S approval, formalin is used to control external protozoa and monogenetic (skin and gill) flukes on finfish, and fungi of the family Saprolegniaceae on finfish eggs. The listed external protozoa are Chilodonella spp., Costia spp., Epistylis spp., Ichthyophthirius spp., Scyphidia spp. and Trichodina spp. The listed monogenetic trematodes are Cleidodiscus spp., Dactylogyrus spp. and Gyrodactylus spp. On penaeid shrimp it is approved against the protozoa Bodo spp., Epistylis spp. and Zoothamnium spp.
Administration and dosing
Formalin acts at the topical site in the water, so the administered concentration and water conditions are the main drivers of efficacy. The FDA-approved concentrations for the control of external parasites on finfish and the prolonged-bath ranges from UF/IFAS are summarized below. Numbers are given as microliters of formalin per liter of water (uL/L), which is numerically equal to parts per million (ppm).
| Setting / species | Concentration | Contact time |
|---|---|---|
| Earthen ponds (all finfish), indefinitely (FDA) | 15 to 25 uL/L | Indefinite |
| Tanks/raceways, salmon & trout above 50 deg F (FDA) | up to 170 uL/L | up to 1 hour |
| Tanks/raceways, salmon & trout below 50 deg F, and all other finfish (FDA) | up to 250 uL/L | up to 1 hour |
| Finfish eggs, fungus control (FDA) | 1000 to 2000 uL/L (up to 1500 for Acipenseriformes) | 15 minutes |
| Aquarium prolonged bath (UF/IFAS) | 12.5 to 25 mg/L (25 mg/L = 1 ml per 10 gal) | Indefinite, aerated |
| Aquarium short-term bath (UF/IFAS) | up to 250 mg/L | 30 to 60 minutes |
UF/IFAS notes that for formalin-sensitive species a half-dose of 12.5 mg/L (about 0.5 ml per 10 gallons) may be used, and that in a flow-through system the water flow should be turned off for at least 12 hours (up to 24 hours) so the chemical maintains contact with the parasite.
Safety and precautions
Formalin removes dissolved oxygen from the water and can deplete it through decay of dead phytoplankton, so vigorous aeration must be provided during every application; UF/IFAS and FDA advise using the lower concentration in ponds, tanks or raceways heavily loaded with phytoplankton or fish. Formaldehyde is a known carcinogen (IARC Group 1) and is handled with appropriate personal protective equipment and a current safety data sheet.
Regulatory status and withdrawal
In the United States formalin is FDA-approved for use as a parasiticide on finfish and penaeid shrimp and as a fungicide on finfish eggs (Parasite-S, NADA 140-989). The FDA has not established a mandatory withdrawal time for formalin; the tolerance for the marker residue para-toluenesulfonamide is 0.90 ppm (900 ppb) in fish muscle and skin (21 CFR 556.118). Legal status, approved species and label conditions vary by country, withdrawal and tolerance rules apply to food fish, and a fish-health veterinarian should be consulted before treating.