A too-common term for many fish "medicines" is "general biocide"... the last word formed from the Latin, "bio" for life, and caedere as in the English "cease"... Formalin is the absolute best example of a biocide... its action is to crosslink (denature) peptides, the building blocks of all "macro" life on our planets (peptides... are attached groupings of of amino acids... larger chains of peptides are called proteins). By cross-linking peptides formalin/formaldehyde makes these chemicals of life not only "dead" as in non-functional, but also prevents their decomposition, as the digestive proteins of decomposers (bacteria, fungi) cannot "fit" onto these newly transformed compounds... All this points up to the utility AND danger of exposure of life to this molecule. It can be a very powerful tool, but it has the double-edged sword of indiscriminate destruction for all life it comes in contact with.
Formalin does have its place... as a disinfectant in retail to commercial livestock settings... a compound of powerful external ectoparasitic properties for dips to extended baths... And even for treatments in designated tanks. Indeed, for some parasitic problems, most notably "Clownfish Disease" (Brooklynellosis) it is the materia medica of first resort. However, formalin is dangerously toxic, to fishes, non-fish life, including you. In all cases, its use should be monitored carefully.
Formalin in stock solutions is made up of 37-40% solutions of formaldehyde gas... That is, 100% formalin solutions are this percentage of formaldehyde dissolved in water (and often with methanol). Hobby "medicines" often contain much lower (diluted) percentages of solution.
Formalin is sold as such in the petfish trade (e.g. Formalin-3 by Kordon Corp), as well as combined with Malachite (e.g. Aquarium Product's "Quick Cure", Kordon Corp.s "Rid-Ich+"). It can be at times/places acquired in full-strength (37%) aqueous solutions through commercial chemical suppliers.
Formalin, but itself and combined with other compounds is useful for treating fungal complaints of fish eggs (more than a day old), fry, adults (e.g. Achyla, Saprolegnia, protozoan (Ichthyophthirius (freshwater "ich"), Costia, Chilodonella, Ambiphyra, Cryptocaryon (saltwater "ich"), Epistylis, Oodinium, Amyloodinium ("velvet"), and Trichodina), and fluke (Gill and body monogenetic trematodes, ) parasites of both marine and freshwater fishes. Formalin is deadly toxic to invertebrates obviously... it MUST NOT be placed in an aquatic system with purposeful non-fish life present.
Definitive diagnoses for these complaints include skin and gill smear observation under a microscope.
Formalin is administered as prophylactic or treatment dips, bath of varying durations, directly to treatment tanks and in flow-through arrangements. The last are rare in petfishing. For dips/baths 125-250 ppm (about 2.7 ml. of 100% soln.)((C1/C2 = V1/V2 -- 100/37 = 1/X) and came up with 2.70mL of 37% stock Formalin solution per a gallon of aerated/temp. adjusted salt water.) per gallon may be used for up to an hour of immersion. Actual time is highly variable, depending on the size, species, physiological state of the fish involved. Such treatments should only be done with you present, added aeration, and a ready hand at netting the treated animals out should they appear over-stressed. Continuous immersion doses should range from 15-25 ppm (0.015 to 0.025 ml.s formalin/l).
Using biocides necessitates not only the removal of host fishes from their diseased system, but either the complete disinfection of these habitats or leaving them "fallow" for a month or more to facilitate the loss of virulence of pathogens in the "parasitized system" itself. In other words, you must need have more than one system to use these chemicals, as returning them to quarters where they've had infectious and/or parasitic disease will "re-load" them with these problems.
Some States allow the sale of full-strength solutions of formalin to the general public (others do not). Some livestock stores (LFS's) and wholesale operations use large enough quantities to justify the purchase/stocking of gallon plus quantities... for preventative dips, net disinfectants and treatments. Check under "chemicals" in your directories for sources here. Kordon Corporation's "Formalin-3" is a 3% solution of formalin. Administered at 1 level tsp. per ten gallons results in a treatment dose of 10 ppm.
Formalin solutions are irritating and volatile... coming "out" of solution if exposed to the air, stinging eyes, nostrils and skin with exposure. It is a known carcinogen, causing cancer in lab animal tests. Medicines containing formalin should be kept out of the light (in opaque bottles) and lot allowed to freeze. People handling formalin should avoid skin contact and breathing fumes.
Use of formalin results in a "triple whammy" of potential hypoxic troubles, with fishes being toxified, producing excess mucus which limits their respiration, outright reduction in oxygen solubility, and death of photosynthetic algae which may be contributing to DO. For these reasons one should NOT treat otherwise too-stressed fishes. Further, you are cautioned in using formalin at higher temperatures than 27 C. (80 F.)... as there is further danger of hypoxia in warm water due to lower dissolved gas and elevated metabolism.
Some groups of freshwater fishes (small characoids/tetras, mormyrids, loaches...) and marines (surgeonfishes, rabbitfishes, dwarf angelfishes...)are sensitive to formalin and formalin/malachite, and should not receive more than half doses of these chemicals. Live plants are also made of proteins... and will die if exposed to formalin. Formalin kills microbes indiscriminately, including nitrifying bacteria. Hence you will need to monitor the accumulation of ammonia and nitrite, and take steps to prevent their exceeding 1.0 ppm (likely by massive changes of pre-conditioned water).
Old, challenged solutions of formalin may form a precipitate of paraformaldehyde... a very toxic, not-useful degraded chemical. Discard any solutions showing a white precipitate.
Use With Other Chemicals:
Formalin can be used in concert with Malachite Green (Leteux-Meyer Mixture), and is often done so for its synergistic treatment effect against ich (freshwater). Know that having both these chemicals together also increases their ichthyotoxicity. Thus you are cautioned not to exceed specified dosages. Hobbyists are encouraged to buy stock Formalin/Malachite solutions (e.g. Aquarium Product's "Quick Cure", Kordon Corp.s "Rid-Ich+"). Commercial interests should see Noga (1996) if they want to make their own stock.
Formalin should not be mixed with potassium permanganate. It is miscible with dechloraminators like Amquel, Acriflavine and copper solutions.
Until there are better, more specific treatment compounds, immunological agents, "nano-tools" that can single out pathogens, undesirable growths, trigger production of beneficial reactants in vivo, we will be left with semi-selectively poisoning our livestock (and ourselves) in the hopes/efforts of "killing off the bad guys, while preserving the good"... The extreme example of the economic use of formalin is a case in point. With careful, metered exposure, this dangerous compound can indeed be useful in the treatment of infectious, protozoan and metazoan undesirables. Its treatment use should be accompanied by careful, continuous observation, added aeration, monitoring of nitrogenous waste accumulation, and ready access to large water changes.
Andrews, Chris, Exell, Adrian and Neville Carrington. 1988. The Manual of Fish Health. Tetra Press, NJ. 208 pp.
Herwig, Nelson. 1979. Handbook of Drugs and Chemicals Used in the Treatment of Fish Diseases. Charles Thomas, Illinois. 272 pp.
Noga, Edward. 1996. Fish Disease. Diagnosis and Treatment. Mosby-Year Book, Missouri. 367 pp.