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Common Name(s): Formalin


Molecular Formula: CH2O
Used For: Treatment of some diseases
Caution: Avoid exposure

Suspected carcinogen

Material Safety Data Sheet: Formaldeyde


An aqueous solution of Formaldehyde is called Formalin. Has been used to treat variety of fish diseases including protozoal ectoparasites, monogenean worms and water moulds on eggs with mixed success. It is volative, irritating and an suspected carcinogen. For treatment of Cryptocaryon it works by interrupting tomot division and is lethal to theronts. Has moderate antibacterial properties, is irritating to fish gills and has algacide properties. Therefore, water has to be well aerated when used. Some fish are sensative to treatment.[1]

It should be stored in the dark and above 4oC to avoid the formation of paraformaldehyde, which is toxic to fish.


Formalin baths are effective for the treatment of Brooklynella, Cryptocaryon and some parasitic flatworms.

Treatment for Cryptocaryon requires a concentration of 25 parts per million (ppm) every second day for two weeks with a complete water change on alternate days. This requires 1 milliliter of formalin in approximately 38 litres (10 gallons) of water. Treatment should occur in a hospital tank is the main system contains invertebrates.[2]

Gloves and goggles should be used when handling.



  1. ( Porritt, M., Cryptocaryon irritans, Reef Culture Magazine, 1.
  2. ( Dickerson, H.W., Treatment of Cryptocaryon irritans in Aquaria, SeaScope, 11, 1994.