Formalin is a saturated solution of formaldehyde, water, and typically another agent, most commonly methanol. In its typical form, formalin is 37% formaldehyde by weight (40% by volume), 6-13% methanol, and the rest water. A quick glance at any commercially available formalin product will confirm these numbers. Formaldehyde provides the disinfectant and bacteriacide/germacide effects of formalin. The water content of formalin provides a dilution of formaldehyde. And the methanol content stabilizes the naturally chemically unstable formaldehyde compound. How formaldehyde “kills” bacteria and germs is discussed in detail below.

Formaldehyde (HCHO) belongs to a class of organic compounds called aldehydes, which are all obtained from the oxidation of alcohol, the most common being methyl alcohol (2CH2OH). During the oxidation process from methyl alcohol to formaldehyde, a certain level of formic acid is produced and will be found in formaldehyde solutions. It is important to note the presence of formic acid as this is a blistering agent most commonly associated with red or fire ants. Further, since formaldehyde is basically unstable in its basic compound form, further oxidation even in storage is possible, thus producing additional levels of formic acid. To help stabilize formaldehyde, methanol is added to the dilute formaldehyde solution formalin. It is very important to note that the formalin compound readily available as a treatment for fish is the EXACT and same compound used in embalming practices. And there is significant irony in this as discussed further below.

Formaldehyde is also highly soluable in water and as such, does not separate or degenerate in water-based solutions. This is one of the main reasons why water is most commonly used to dilute formaldehyde into the common formalin compound. Additional reasons for using water in the formalin compound include cost and the ability of water to mix with other chemical agents readily. This point will have more relevance when discussing the combined use of formalin and salt in fish treatments.

While formaldehyde is a potent disinfectant and anti-bacterial agent, it is essentially ineffective as a fungicide, insecticide, or larvacide. This is an important point to remember when considering formalin in the treatment of fish. While formalin will work for such problems as gill flukes, surface infections, and other parasites, it will NOT work on argulus, fish lice, and other macro-parasites that we associate with treatments requiring organophosphates, such as dimlin. Nor will formalin be effective against mold and fungus-related problems, such as saprolegnia.

But before you go thinking formalin is an ideal anti-bacterial treatment, first consider how formaldehyde “kills.” Unlike most anti-bacterial and germicidal agents which poison the bacteria and germ cells, formaldehyde kills cell tissue by dehydrating the tissue and bacteria cells and replacing the normal fluid in the cells with a gel-like rigid compound. The latter effect exhibits the coagulation properties of formaldehyde. Tissue and bacterium cells are made of protoplasm and as such, contain large amounts of moisture. The introduction of formaldehyde into the tissue dries out the protoplasm and destroys the cell. In terms of embalming practices, this is a perfect situation as the formaldehyde not only disinfects the tissue but replaces the tissue cell moisture with a rigid gel thus allowing the embalmed tissue to maintain its contour. Additionally, the “new” cell structure will resist further bacterial attacks as its composition now contains a formaldehyde-based compound. So, while the usual list of anti-bacterial agents, such as tetracycline, amikacin, baytril, and the like poison their respective bacterial enemies and are then flushed from the system by the kidneys and liver, formalin is retained in the now altered tissue structures of the living organism.

As stated previously, formalin was originally designed for the purposes of disinfecting and preserving tissue in embalming practices. It was not originally contemplated for use in fish medicine. Since formaldehyde is highly soluble in water, this combination offered a near perfect solution for easy permeation of tissue and cell structures. This is an important point to consider when using formalin on fish as the fish will “absorb” a certain level of formalin into its tissue and cell structures just by the very nature of how fish process water in their environment. This is where the principles of osmosis are important.

Osmosis or osmotic pressure is the passage of a solvent through a membrane separating two solutions of different densities. The basic rule of osmosis is that fluids will flow from the less dense environment to the more dense environment through the membrane. Using a practical example, consider the use of salt in koi ponds. Here, we need to consider the salt content of the fish itself and for koi and this level is about .9%. The pond water, on the other hand, maybe anywhere zero percent salt content up to whatever level the pond owner has manipulated it. There is much debate on the use of salt in the treatment of many koi problems. While there is no question that a salt level of .3% will effectively rid the pond and fish of most micro-parasites, it is the effect of salt on the osmotic regulation process of the fish that most ponders believe is a good stress reducing regimen. So as an example, the pond has a salt level of .15% and the fish’s level is .9%, then the osmotic pressure or flow of water is from the pond and into the fish’s body. And the same can be said for any salt level up to .9% where the equilibrium of osmotic pressures will start causing severe problems with the fish’s physiology. In fact, if the salt levels of the pond become equal to or greater than that of the fish, the fish can actually suffer dehydration as the flow of the fluids reverses based on reversed fluid densities.

But the driving factor behind the need to control osmotic regulation and relieving the fish of added stress is not specifically with the addition of salt to the water, but what happens to the “mechanics” of the water once the salt is added. As most of us remember from high school chemistry, every fluid has a surface tension.