formalin

             Formalin   (Formaldehyde solution,)

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  be protected from air.

Formaldehyde is most effective at ridding fish of common parasitic protozoan Black Spot, Clownfish, and White Spot marine ich infestations, as well as flukes, lice, and fungal diseases that both fresh and saltwater fishes can contract. Although a toxic chemical that is hazardous in its pure form, by purchasing and using a standard over-the-counter "formalin" product, typically a 37% solution of formaldehyde diluted with water, it's rather easy to treat ich diseased fish.

• Set up a quarantine tank,providing it with vigorous aeration. Even though hyposalinity does not cure ich infestations, lowering the specific gravity of the QT water to about 1.010 ppm can help to keep any possible new free-swimming organisms from reinfecting the fish.
• Prepare a container with treatment water, mixed per the directions on a
  manufacturer's formalin product. Include vigorous aeration of the treatment water, such as inserting an air stone. Because harmful ammonia can build-up in the water during treatment, it is recommended to add AmQuel to avoid complications that can occur from ammonia burn. 
• Prepare another container, preferably with lowered salinity 1.010 ppm water. This will be used to give fish a quick dip in after they have been taken out of the treatment water, which will assist in removing weakened or dead parasites, mucus or other unwanted organic matter from the fish before placing them into the QT.
• When all is ready, place the fish into the formalin treatment container, proceeding per the instructions for the formalin product you are using, and by referencing the dip and bath timing and other important tips below

• Once the treatment is finished, depending on the stress level of any particular fish at this time, give each a quick 30-second, one or two-mintue dip in the second container of water, and then place them into the QT.
• All fish should remain in the QT for at least 4 weeks, and during the quarantine period, you should continue treatment with formalin as instructed. You should also treat preventatively for potential secondary bacterial infections with an appropriate antibiotic or antibacterial medication. Make certain to leave the main aquarium void of fish. This will allow the protozoa to run their life cycle and die off, and prevent reinfestation.