of Formalin to Control Fish Parasites
By: Ruth Francis-Floyd
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
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
- 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 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
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
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
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.
Ruth Francis-Floyd, extension
veterinarian, Department of Large Animal Clinical Science, Cooperative
Extension Service, Institute of Food and Agricultural Sciences, University
of Florida, Gainesville, 32611.
This document is copyrighted by the University of Florida, Institute
of Food and Agricultural Sciences (UF/IFAS). Date published April