How to Treat Diseased Marine Fish With Formalin

Close-up of a clown fish swimming in its container

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Formaldehyde is most effective at ridding fish of some common parasitic protozoan, such as Clownfish disease (Brooklynella), and Cryptocaryon (White Spot) marine ich infestations, as well as flukes, lice, and fungal diseases that both fresh and saltwater fishes can contract. Although formaldehyde is a toxic chemical that is hazardous in its pure form, a 37 percent solution of formaldehyde diluted with water (formalin) is available commercially through pet stores to treat diseased fish. It must still be handled carefully as it is toxic if ingested or gets onto the skin.

What You'll Need

  • QT (a Quarantine Tank)
  • 37 percent formalin solution
  • Fresh and saltwater
  • 2 treatment containers of choice
  • AmQuel, or another suitable ammonia neutralizing product
  • Antibiotic or antibacterial medication

Here's How

  1. Set up a quarantine tank, providing it with vigorous aeration. Even though hyposalinity does not cure parasitic infestations, lowering the specific gravity of the QT water to about 1.010 can help to keep any possible new free-swimming organisms from reinfecting the fish.
  2. 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 formalin will decrease the oxygen in the water. Harmful ammonia can also build-up in the water during treatment, so it is recommended to add AmQuel or similar ammonia neutralizing products to avoid complications that can occur from ammonia burn
  3. Prepare another container, preferably with lowered salinity 1.010 specific gravity 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.
  4. 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.
  5. Once the treatment is finished, depending on the stress level of any particular fish at this time, give each a quick 30-second to two-minute dip in the second container of water, and then place them into the QT.
  6. 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 on the package label. 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.


  • Dip timing: Fish in the advanced stage of this disease will already be very weak and stressed, and most likely on the verge of dying. An animal in this condition may only be able to tolerate a full-strength treatment for a few minutes, not at all, or might die during the procedure. If this is a concern, by diluting down the recommended formalin dosage for treatment, an extremely ill fish might hold up better as well as withstand longer exposure time.
  • Bath timing: In all likelihood, a fish that is still in rather fair health should be able to withstand a minimum 15-minute bath. If no signs of distress arise after 15 minutes, continue and extend the treatment to 30, and if possible up to 60 minutes.
  • Overall treatment: Do not leave fish unattended at any time during a dip or bath treatment of any kind. Watch them closely, and if any fish show signs of distress, remove it immediately.
  • Urgent treatment: If a formalin solution is not available for immediate use, temporary relief may be provided by giving fish a freshwater dip/bath. Even though this treatment will not cure the disease, it can help to remove some of the parasites and reduce the amount of mucus in the gills to assist with respiration problems. Once the initial dip/bath is done, place the fish into the QT under hyposalinity treatment, and obtain a formalin medication as soon as possible to begin treatment.


  • Although formaldehyde is effective for treating external protozoan infestations, as well as fungal diseases, overdosing or exposing fish to this medication too frequently may cause additional injury and even death. It is harmful to invertebrates, and should not be added to the main aquarium. It should only be used in a QT, and for making up treatment dip/bathwater.