Formalin

Formalin is a clear aqueous solution of formaldehyde and methanol, commonly used as a preservative[1]. It is a liquid comprised of formaldehyde gas saturated in water, with "pure" formalin made of approximately 37% formaldehyde gas resulting in 100% active ingredient[2]. Formalin solutions for use on fish contain 10 to 15% methanol, which inhibits the formation of paraformaldehyde, a highly toxic compound[2].

Formalin is used as a bath treatment to control parasitic infections on the external surfaces (skin, fin, gills) of fish, being extremely effective against most protozoans as well as some larger parasites[2]. It is not the preferred treatment for external bacterial or fungal infections on fish, but it is used to control water mold ("fungi") on fish eggs[2]. Formalin is not effective against internal infections of any type[2].

The use of formalin requires caution due to the hazardous nature of formaldehyde, which is a known carcinogen[2]. It should only be handled by trained personnel wearing appropriate personal protective equipment (PPE) such as gloves and eye protection[2]. A current safety data sheet (SDS) should be available at all times and carefully read before use[2]. Formaldehyde is a noxious gas, and formalin must be kept in a sealed container in a well-ventilated area to prevent exposure to fumes, which may result in headaches and irritation to eyes and respiratory tract[2].

Formalin can be used as an indefinite bath at a concentration of 12.5 mg/l to 25 mg/l or in a short-term bath (30–60 minutes) at high concentrations of 1000 to 2000 mg/l to control water-mold ("fungal") infections on fish eggs[2]. However, appropriate management practices must be implemented to prevent recurrence[2].

The toxic effects of formaldehyde and formalin include death suddenly when swallowed, and the introduction of formalin into the stomach is followed by the production of a gastritis which varies greatly in character[3]. Intraperitoneal injections of formalin cause peritonitis of a fibrino-haemorrhagic character, and the lethal dose of formalin when injected intraperitoneally into guinea pigs is approximately 2 cc. of 1–1000 formalin for each 100 grm. of body weight[3].

Formalin is also listed by the CDC as a hazardous substance, with exposure limits and recommendations for personal protection and sanitation[4].

Citations:

[1] https://www.merriam-webster.com/dictionary/formalin

[2] https://edis.ifas.ufl.edu/publication/VM061

[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2124506/

[4] https://www.cdc.gov/niosh/npg/npgd0294.html

[5] https://en.wikipedia.org/wiki/Formaldehyde