Formalin Baths and Freshwater Dips for Marine Parasites: Doses and Safety
Formalin baths and freshwater dips are proven tools against external protozoa and flukes on marine fish. Learn the sourced concentrations, dip durations, the mandatory aeration and temperature precautions, and why formalin must be handled as a carcinogen.
What formalin treats
Formalin is approximately 37% formaldehyde gas dissolved in water. It is extremely effective against most protozoan parasites and some larger parasites such as monogeneans (flukes) on marine fish, which makes it a mainstay of quarantine-tank parasite control.
Doses
For a prolonged or indefinite bath, use 12.5 to 25 mg/L (about 1 mL of formalin per 10 gallons). For a short-term dip, use up to 250 mg/L (about 1 mL per gallon) for 30 to 60 minutes. Always treat in a hospital tank and watch the fish closely throughout.
Freshwater and salt dips for flukes
For monogeneans on marine fish, a freshwater dip of 1 to 5 minutes (depending on the species' tolerance) osmotically dislodges the parasites; a 3% (30 g/L) salt dip under 10 minutes is an alternative. Remove the fish immediately if it rolls over or loses equilibrium. Matching the dip water's temperature to the display is good practice.
Safe procedure
- Set up a hospital or dip container with vigorous aeration.
- Prolonged bath: 12.5 to 25 mg/L formalin; or short dip: up to 250 mg/L for 30 to 60 minutes.
- Never treat if the formalin or system is below 40°F (5°C).
- Freshwater dip for flukes: 1 to 5 minutes; remove the fish the moment it rolls.
- Handle formalin as a carcinogen: gloves, eye protection and ventilation.
Sources: ask.ifas.ufl.edu (UF/IFAS VM061); www.merckvetmanual.com