Formaldehyde: Friend or Foe?

Formaldehyde: Friend or Foe?
by Terry D. Bartelme

Which is worse, the cure or the disease? When considering aquarium inhabitants, the answer to this question lies largely in the method used for delivery of the therapy. Many agents are indeed very dangerous, and must be used correctly and with extreme caution. Formaldehyde or formalin is one of these chemicals that can be effective when used properly or deadly if used incorrectly. Formalin works as a preservative by denaturing proteins. It can be used as embalming fluid or as a lifesaving agent by killing pesky parasites. An aqueous solution of 37% to 40% formaldehyde gas equals 100% formalin. Although precautions must be used while administering therapy with this agent, it is very effective for treating a number of external parasite infections. Formaldehyde is a potent germicidal against all organisms, and it has applications as an algecide, fungicide, protozoacide, antihelmintic, and crustacide.
Formalin is carcinogenic and irritating to the skin, so gloves should be used when administrating treatment. Formalin must be stored in a tightly sealed container and used only in a well-ventilated area to avoid breathing the vapors or irritating the eyes. It should be stored in the dark above 30 degrees Fahrenheit. Methanol is added to formaldehyde to prevent the formation of paraformaldehyde in storage, and formalin should not be used if this white precipitate is evident. Paraformaldehyde is highly toxic to fish and can kill them on contact. It is best to strongly aerate the formalin for several hours prior to using it in a dip. This will remove as much of the methanol as possible, because the methanol is also toxic to fish. Please be careful to keep formaldehyde out of the reach of children and animals at all times, and be sure to dilute the used water after treatment before carefully disposing of it.

Formaldehyde can be used successfully to treat Brooklynella, Turbellarian, Uronema, and monogenetic trematode infections of the gills, fins, and skin of fish. The variables for treating with formaldehyde are the concentration of the chemical or the dosage, temperature of the water, and condition of the fish. Of course time is of the essence when treating any disease, infection, or condition. Therapy is most effective before the infection becomes advanced, and formaldehyde is contra-indicated when fish have sores, wounds, or gill disease. The treatment should not be carried out on fish that have been recently fed. Avoid using this treatment on recently transported or moved fish, because they will not have the normal degree of mucus layer protection
Formaldehyde can quickly kill the bio filter in an aquarium and is highly toxic to invertebrates. Therapy should be carried out as a short-term bath in a large plastic bucket or barrel. Formalin can be purchased at many tropical fish stores or pharmacies. Look for a 37% to 40% solution. Whenever possible avoid removing the fish from water when transferring it to and from the dip to prevent unnecessary stress. Corralling the fish into a container instead of using a net will reduce the likelihood of injury.
A series of three dips, one every other day, should be performed for 45 minutes each time to ensure that any parasites hidden under the mucus layer or imbedded in the external tissues are eradicated. Never extend treatment beyond 60 minutes. The dosage is 20 drops or I milliliter of formalin per gallon of water, or 250 parts per million formalin. Formalin will remove oxygen from the water so it is very important to place an air stone or other device in the dip to provide a constant supply of' oxygen. Good water will guarantee that there are no areas in the dip with higher concentrations of the drug. A small heater can be placed in the dip to maintain a stable temperature during treatment. Formaldehyde becomes more toxic as the temperature rises. Also the amount of' oxygen that water can hold is at higher temperatures, so I recommend not using, formalin above 80 degrees Fahrenheit. The fish should be carefullv attended throughout treatment, and immediately removed to untreated water if there are signs of extreme stress such as jumping, excess mucus-production, faded color, or loss of equilibrium. An alternate method of administration is a prolonged bath in a quarantine tank at 15 to 25 mg/liter. Two drops per gallon of water is roughly equivalent to 25 mg/liter.
When treating fish for any sort of parasite infection you should be aware of the possibility of a secondary microbial infection at the site of attachment. If you find that you need to treat for bacterial infection an antibiotic should be administered in a separate container or aquarium tank. A couple of good choices for treating external bacteria infections caused by parasite wounds are Neomycin and Nitrofurazone. Neomycin should be used as one-time dose on the first day of 250 mg per gallon of water with treatment lasting three to five days. Do not use Neomycin in a crowded aquarium or quarantine tank. Although it is a very effective antibiotic, it will quickly destroy the bacteria that perform biological filtration. This agent should not be used simultaneously with copper or sulfa drugs.
Nitrofurazone can be used at a one-time dose of 40 to 50 mg per gallon with treatment also continuing for three to five days. Nitrofurazone is easier on the biological filter than Neomycin and has the added benefit of being absorbed well by fish for cased of internal bacterial infection. These two antibiotics can be safely combined for a synergistic effect.
Although formalin can be used simultaneously with copper or malachite green I do not recommend it. Malachite green is very toxic to small fish, and concentrations of this chemical have been known to vary from the labeled description. Copper is largely ineffective for treating infections of Brooklynella, monogenetic trematodes, and some other external parasites apart from Cryptocaryon and Amyloodinium.
Another means of treating marine fish for external parasites is a freshwater dip. Although a freshwater dip may not be as effective for removing some parasites, it is a viable alternative when you need to treat the fish quickly and formalin is not readily available. The change in osmotic pressure during a freshwater dip can destroy exposed parasites or simply cause them to detach from the fish. This procedure forces marine fish to attempt to osmo-regulate in a manner that is reverse to what is natural. The internal fluids and blood of marine fish generally have a salinity of 11 parts per thousand, or a specific gravity of 1.008. Because freshwater dips can be very stressful they must be used with some degree of caution. Many fish will react in a negative manner, so they should be closely monitored during treatment for signs of abnormal stress such as loss of balance without quickly returning upright. If a fish exhibits signs of extreme stress it should be placed back into saltwater immediately. The water should be dechlorinated prior to use and care should be taken to adjust the temperature and pH to match closely the water the fish are taken from. Treatment can last anywhere from 1 to 20 minutes, depending on how the fish respond.
It is my preference to use formalin rather than a freshwater dip in most instances, because I find it to be a more effective and less stressful therapy. Although formaldehyde certainly is not a cure-all for every type of parasite infection, it can be an effective tool to aid your fish toward recovery and good health.