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The chemical compound formaldehyde (H2CO) (also known as methanal, formaldehyd, formaldeído ) is a gas with a pungent smell.

Although formaldehyde is a gas at room temperature, it is readily soluble in water. It is most commonly sold as a 37-56% aqueous solution with trade names such as formalin or formol. In water formaldehyde converts to the hydrate CH2(OH)2. Thus formalin contains very little H2CO.

Formaldehyde is readily oxidized by atmospheric oxygen to form formic acid. Formaldehyde solutions should be protected from air.

It is also used as a preservative in vaccinations. In medicine, formaldehyde solutions are applied topically to dry the skin, such as in the treatment of warts.

Formaldehyde is also used as a preservative of tissue. Formaldehyde based solutions are used in embalming to disinfect and temporarily preserve human or animal remains pending final disposition.

  • Toxicity of formaldehyde increases with an increase in the aquarium water temperature.

In the aquarium hobby it is most often used as an aqueous solution, used as a disinfectant as it kills most bacteria.

  • This chemical compound is a health hazard if you are repeatedly exposed to it and people have developed allergies to it. See links below for details.

In the aquarium hobby it is most often used as the form Formalin and is commoningly mixed with other chemicals like Malachite green for example, as an additional aid in killing bacteria and fungus. It is often added to water conditioners.

  • Formalin should be stored in the dark and above 4C (39F) to prevent the formation of paraformaldehyde.
  • Formalin is used against protozoan and metazoan parasites.
  • Formalin binds to oxygen so aerate the water without stressing ill fish.[1]
  • Formalin will kill weak or struggling for breath fish.[1]
  • Formalin will kill filter bacteria.[1]

Typical treatments with Formaldehyde/Formalin[edit]


  1. 1.0 1.1 1.2 Formalin treatment by Dr. Erik Johnston