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This is basically a 37% solution of formaldehyde and water. It is a potent external fungicide, external protozoacide, and antiparasitic, and seahorse keepers commonly use formalin to cleanse new arrivals of ectoparasites during quarantine. Formalin (HCHO) is thus an effective medication for eradicating external parasites, treating fungal lesions, and reducing the swelling from such infections. It is a wonder drug for treating cases of Popeye caused by trematodes, and also eradicates nematodes as well as bacteria.
Formalin has a limited shelf life and degrades to the highly toxic substance paraformaldehyde (identified as a white precipitate on the bottom of the solution); avoid using any formalin product that has such a precipitate at the bottom of the bottle.
When using formalin, beware that it basically consumes oxygen so vigorous aeration must be provided during treatment. It can be administered as a 1-hour bath at a concentration of 200 ppm (l ml per 5 liters of seawater or about 1/5 teaspoon per 1.33 gallons).
Formalin is guaranteed to knock your biofilter for a loop, so confine its use to the hospital tank only!