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Formaldehyde sol. 37% vol.



Formalin is a colourless transparent liquid with characteristic sharp odour and specific gravity 1.09kg/lt. (20°C).

It is a virtually saturated solution of formaldehyde in water. At normal water temperatures it contains 34-38% w/w formaldehyde. Usually, it contains 12% methanol to slow down the polymerisation of the formaldehyde.


Formalin is a widely used anti-parasitic agent, effective against many protozoa and monogenetic fluke parasites of fish skin and gills. It is also active against many bacteria, such as the Gram negative filamentous bacteria infecting the fish gills. Formalin is safe even for small fish when used in baths according to proper protocols.

Apart from its therapeutic use, formalin is used also for prophylaxis against the ecto-parasites in order to prevent the seasonal build up of parasitic loads on caged fish, or the proliferation of the parasites on valuable brood fish in hatcheries.

Formalin dilutions may be used as a strong disinfectant in hatcheries.


It is recommended that a light dilution of formalin (1:100) is prepared and gradually dispersed in the fish tank or tarpaulin until the required bath concentration is achieved.

Diluted Ο2 should remain at the safe levels of 6 to 8ppm. Formalin baths should last up to 60 minutes and may be repeated according to veterinary instructions, depending on the season and the type of parasitic infestation under treatment. Formalin baths are not recommended when the water temperature exceeds 26°C.

Diluted solutions are normally designated by their content of formalin, not of formaldehyde; for example a use concentration obtained by diluting formalin 1:10,000 will be designated 0.01% formalin or 100ppm formalin but actually contains about 37μg/ml (ppm) formaldehyde.

The maximum safe formalin concentrations in baths differ according to the age/size of the fish and the level of dissolved oxygen in the water. Formalin is a reducing agent, which will absorb oxygen from water; in addition it irritates the gills and lowers the efficiency of osmoregulation and gaseous exchange at the gill epithelium. Besides, when formalin baths are applied, the bacteria and/or the parasites, which the baths are meant to destroy, have already impaired the gill function. Therefore, water aeration, or preferably water oxygenation, should be provided throughout a formalin bath.

Indicative formalin concentrations for hourly baths (sea bream and sea bass):

· Fry 0.3g-0.5g: 30ppm  · Fry 0.5g-1.5g: 50ppm  · Fingerlings 1.5g-3g: 80ppm
· Fingerlings 3g-20g: 120ppm  · Caged fish >20g: 150ppm  · Brood-stock: 200ppm.

Precautions (for operator safety):

Undiluted formalin is toxic and caustic for the skin and eyes as well as for the upper respiratory tract when fumes are inhaled and the gastrointestinal tract if accidentally swallowed. In the eyes formalin is very irritant, even in considerable dilution. Always flush exposed eyes or skin with plenty clean water for several minutes. If accidentally swallowed, the patient should drink plenty of water, milk or demulcents taken. Vomiting should not be induced and subsequent to such first aid measures proper medical help should be sought after (gastric lavage).

When handling formalin apply safety measures and use proper equipment. Keep away from children.

Packaging and storage:

Formalin is usually distributed in sealed plastic containers of 30kg (27.5 lt.).

It should be stored in a ventilated, dark place, away from oxidising agents. Storage temperature should exceed 10°C. (Polymerisation of formaldehyde to the toxic polymer, paraldehyde, occurs faster in sunlight and at low temperatures.)




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Every effort has been made to ensure that the information above is accurate until the date of last editing. It is based upon the accumulated experience of applied veterinary work. The information may be used as a guide and is not meant to replace professional veterinary guidance for fish treatment. The author cannot take responsibility for incorrect interpretation or any resulting consequences.

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