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Tilapia Topic: Disease Treatment:


Information about Formalin Treatment

Formalin is effective as a bath treatment for most protozoan external parasites and monogenetic trematodes. Formalin bath treatments are of little use for treatment of external bacterial or fungal infections. Formalin is algicidal, but is not applied for algae control.

Formalin is solution of 37% formaldehyde, but for dosage calculations in the treatment of fish parasites formalin solution is considered as 100% active. Formalin solution is soluble in both fresh and marine water.

The toxicity of formalin to fish increases with temperatures above 22oC, thus dosage for a one hour bath exposure should not exceed 167 ppm. Higher dosages up to 250 ppm may be attempted, but caution is advised and reduced exposure (30 to 45 minutes) times are indicated.

Formalin removes oxygen from the water at the rate of 1 ppm O2 for each 5 ppm formalin applied. Thus, adequate aeration should be used when formalin treatment is applied. In addition, during treatment with formalin fish should be evaluated every few minutes for signs of respiratory distress (gasping at the surface, forced heavy respiration, etc.) and if present, the formalin solution should be flushed out immediately.

Formalin is a clear solution. Prolonged storage, exposure to direct sunlight or temperature below 5oC of formalin results in the formation of paraformaldehyde. Paraformaldehyde is highly toxic to aquatic life. Thus, formalin that appears cloudy, has a white precipitate on the bottom of the container (to visualize swirl the bottle to suspend the precipitate) or has been in storage for a prolonged period should not be used for treatment of fish.

Formalin is approved by the FDA for use in food fish. However, this approval applies only to specific brands of formalin solution. Be sure the formalin used for treatment of fish parasites is an approved form (ie. Paracide F from Argent Laboratories).

Forms of Formalin Used

Formalin is a liquid solution that is 37% formaldehyde by weight , but for purposes of external parasite treatment of fish is considered 100% active chemical. Certain commercial preparations of formalin have been approved by the FDA for use with food fish.

The user is advised to only use an approved formalin solution for treatment of parasites on fish intended for human consumption.

Other circumstances that influence formalin treatment

  1. The potential for formalin toxicity increases if the fish have significant gill disease.
    Warning: It is dangerous to apply formalin to fish with gill disease and it is necessary to have air addition and continuous monitoring of the fish during Rx.
  2. Formalin solution should be stored at temperatures above 4.4oC (40oF), because lower temperature promotes the formation of paraformaldehyde. Also, prolonged storage of formalin promotes the formation of para- formaldehyde.
    Suggestion: Formalin should be stored at room temperature, and the formalin for fish disease treatment should not be stored longer then one year.

Contraindication for using formalin in bath treatment

  1. Formalin removes oxygen from water at a rate of approximately 1 ppm O2 for each 5 ppm of applied formalin. Thus, be sure to provide ample supplemental aeration during and after the period of formalin treatment. Observe fish every few minutes during formalin treatment and flush the formalin solution from the container if fish display signs of respiratory distress (gasping at the surface, disorientation, etc.).
  2. Due to the effects on oxygen, formalin is more toxic when water temperature is 21ø or higher. Thus, increase aeration or decrease formalin dosage when treatments are done in warmer water.
  3. Formalin is algicidal. Do not use if high algal density is present in the water.
  4. Formalin is formaldehyde dissolved in water. Formaldehyde will slowly be converted to paraformaldehyde during storage. Paraformaldehyde is quite toxic to fish and formalin that contains paraformaldehyde should never be used as a treatment. Paraformaldehyde forms a white precipitate in the otherwise clear formalin solution. Thus, when paraformaldehyde is present a fine white powder will be on the bottom of the container or, if shaken, the formalin solution will turn cloudy. Never use a formalin solution for treatment of fish that is cloudy or has a white precipitate in the bottom of the bottle.

See also:

file: /Techref/other/pond/tilapia/formalin.htm, 5KB, , updated: 2007/2/5 19:20, local time: 2017/11/21 07:50,

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