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Use of Formalin to Control Fish Parasites
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APPLICATION OF FORMALIN AS A PARASITICIDE FOR FISH
Formalin is applied as a bath treatment. It can be applied as a prolonged bath, which means it is placed into the water indefinitely, or it can be applied as a short-term bath, which means fish are placed into the bath for a relatively short period of time (30 to 60 minutes) and then placed into clean (untreated) water. The concentration of chemical used is determined by the period of time the fish are to be in contact with the chemical, the temperature of the water, and the condition of the fish. Extremely sick fish may not be able to tolerate a "full" treatment. Any time fish exhibit signs of distress (i.e., darting, gasping, or trying to jump out of the water) during a chemical treatment, they should be placed into clean (untreated) water at once.

The concentration of formalin appropriate for a prolonged bath is 15 to 25 mg/l. The lower concentration, 15 mg/l, would be appropriate for pond use, however, the use of formalin in ponds is discouraged for several reasons which are discussed below. The higher concentration, 25 mg/l, is easily applied to aquaria and tanks at 1 milliliter (ml) per 10 gallons, or 2 drops per gallon. These are quick and easy ways of measuring this concentration. Any time formalin is applied, vigorous aeration must be provided.

For short-term baths a concentration of 250 mg/l, or 1 ml per gallon, can be delivered for 30 to 60 minutes. At moderate water temperatures (less than 70°F or 21°C), fish can be left in a 250 mg/l formalin bath for about one hour; however, if fish are weak or noticeably sick, the treatment should be discontinued after 30 minutes. Never exceed one hour of chemical exposure at this concentration. If fish show signs of distress before the allotted time has elapsed, they should be removed from the treatment immediately. At warmer water temperatures (greater than 70 0 F or 21 0 C) the concentration of formalin should be decreased to 150 mg/l for no more than one hour. Vigorous aeration must be provided to fish during treatment.

If you are uncertain how to calculate the amount of formalin needed to treat your system, contact your IFAS county extension agent for assistance.


USE OF FORMALIN IN FISH PONDS
Although formalin has historically been used to control protozoan infestations of fish in ponds, its use in aquaculture ponds is generally discouraged. First, it is quite expensive, and large volumes are needed to treat even a small pond. Other, less expensive chemicals, such as potassium permanganate, are available which have the same spectrum of activity as formalin, but are more cost effective for commercial use. Second, formalin chemically removes dissolved oxygen from water, and this action, along with its algicidal activity, creates a situation which is conducive to development of an uncontrollable oxygen depletion. The direct cost and risk associated with use of formalin in fish ponds make its use in ponds difficult to justify.


USE OF FORMALIN IN HAULING BOXES
Any time fish are moved from one facility to another, there is concern about the potential spread of disease between populations. The potential spread of many protozoan diseases can be eliminated by treating fish for parasites while they are still in the hauling box. Ideally this should be done before they are transported, but if that is not possible, treatment of fish in the hauling box is a reasonable option.

To provide a formalin treatment to fish while they are still in the hauling box, water in the box must be tempered so that it is similar to receiving water in terms of temperature and pH before the treatment is started. When this is done properly, the treatment may be halted at any time by simply opening the box and releasing the fish into the pond. In addition, constant vigorous aeration must be available during the treatment. If these precautions have been taken care of, formalin can be applied as it would be for any short-term bath (150 to 250 mg/l based on water temperature for 30 minutes). Remember: never leave fish unattended during treatment, and never leave fish in the formalin bath for more than 60 minutes.


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