Use of Formalin to Control Fish Parasites 1
INTRODUCTION - WHAT IS FORMALIN?
Formalin is a generic term which describes a solution of 37% formaldehyde
gas dissolved in water. Solutions of formalin for use on fish should contain
10 to 15% methanol, which inhibits formation of paraformaldehyde (discussed
below), a highly toxic compound. Two commercial products have been approved
for use in aquaculture by the Food and Drug Administration (FDA). These are
Formalin-F sold by Natchez Animal Supply, Natchez, Miss. and Paracide-F, sold
by Argent Chemical Laboratories, Redmond, Wash. Both of these products have
been approved by FDA for use on food fish (trout, salmon, catfish, largemouth
bass and bluegill) as a parasiticide. There is no legal withdrawal time (time
after the chemical was used before fish can be slaughtered for food) for
either of these products.
HOW IS FORMALIN USED IN AQUACULTURE?
Formalin is used as a bath treatment to control external parasitic
infections of fish. It is extremely effective against most protozoans, as well
as some of the larger parasites such as monogenetic trematodes. Formalin
effectively kills parasites on gills, skin, and fins. It is not the preferred
treatment for external bacterial or fungal infections. In addition, high
concentrations of formalin are used to control fungi on fish eggs. Formalin is
not effective against internal infections of any type.
SPECIAL CONCERNS REGARDING THE USE OF FORMALIN
Concerns for safety of personnel
- Formaldehyde is a known carcinogen. It should only be handled by
personnel wearing protective clothing such as gloves.
- Formaldehyde is a noxious gas. Formalin must be kept in a sealed
container in a well-ventilated area. Exposure to fumes will result in
irritation to eyes and respiratory surfaces.
- Some people develop a sensitivity to formalin over a period of time
which involves repeated handling of the chemical. These individuals should
avoid handling the chemical.
Concerns for safety of fish
- Formalin chemically removes oxygen from the aquatic environment. Each 5
mg/l of formalin applied removes 1 mg/l of dissolved oxygen. This is one
reason why use of formalin in ponds is discouraged.
- Formalin is an algicide. When applied to pond water, formalin kills a
portion of the algae present, thereby reducing the ability of the algae to
produce oxygen through photosynthesis. Further decreases in oxygen in the
pond can result as the dead algae decompose.
- Formalin should be stored in an area where it is protected from extremes
of heat and cold. Never use formalin when storage temperatures fall below
40°F (5°C) or when a white precipitate (powder) is present. At cold
temperatures formaldehyde is transformed into paraformaldehyde (white
precipitate), a highly toxic material which will kill fish on contact.
- Formalin toxicity is increased at high water temperatures. If water
temperatures exceed 70°F (21°C), the concentration used should be decreased.
- When treating parasites on sensitive species, such as hybrid striped
bass, the concentration of formalin delivered in a prolonged bath should not
exceed 10 mg/l.
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
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.
USE OF FORMALIN IN HATCHERIES TO CONTROL FUNGUS ON EGGS
Formalin is approved by FDA for control of fungi in fish hatcheries.
Concentrations of 1000 to 2000 mg/l can be applied to fish eggs for 15 minutes
to aid in control of fungus. Hatchery managers are reminded that sanitation is
important for the prevention of fungal invasions on fish eggs. Dead eggs
should be removed from the system promptly because they serve as a source of
infection for adjacent, healthy eggs.
Formalin is a liquid formulation of 37% formaldehyde gas dissolved in
water. Two brands of formalin, Formalin-F (Natchez Animal Supply, Natchez,
Miss.) and Paracide-F (Argent Chemical Laboratories, Redmond, Wash.), have
been approved by FDA as parasiticides for use on fish. Formalin is effective
against many external parasites, including protozoans and monogenetic
trematodes. It is not generally considered the best treatment for external
fungal or bacterial infections. Formalin can be delivered in a short-term bath
at a concentration of 250 mg/l - or 150 mg/l if water temperature is greater
than 70°F (21°C) - for no more than 60 minutes. It can be delivered as an
indefinite bath at a concentration of 15 to 25 mg/l. Formalin is an excellent
parasiticide for use in tanks and aquaria, but its use in ponds is discouraged
because it chemically removes oxygen from the water and can contribute to
catastrophic oxygen depletion under pond conditions. This is avoided in tanks
and aquaria by always supplying vigorous aeration when formalin is used. High
concentrations of formalin (1000 to 2000 mg/l for 15 minutes) can be used to
control fungal infections on fish eggs; however, appropriate management
practices must be implemented to prevent recurrence.
1. This document is VM-77, one
of a series of the College of Veterinary Medicine, Florida Cooperative
Extension Service, Institute of Food and Agricultural Sciences, University of
Florida. Date published April 1996. Please visit the FAIRS Web site at http://hammock.ifas.ufl.edu.
2. Ruth Francis-Floyd, Joint
Associate Professor, Department of Large Animal Clinical Sciences and
Department of Fisheries and Aquatic Sciences, Cooperative Extension Service,
Institute of Food and Agricultural Sciences, University of Florida,
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Florida Cooperative Extension Service / Institute of Food and Agricultural
Sciences / University of Florida / Larry R. Arrington, Interim Dean
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