Formalin (37% formaldehyde)

What It Treats Marine Velvet Disease (Amyloodinium), Brooklynella, Uronema, Flukes (Monogeneans), Black Ich (Turbellarians).

How To TreatIn order for Formalin to be fully effective, the product you are using should contain 37% formaldehyde. At present time there is only one aquarium product which meets this criteria: Parasite-S (MSDS is here) from Syndel USA:

You can also use 37% UPC lab grade Formaldehyde (Amazon sells it here – also contains 14-15% Methanol which is OK), following these dosing instructions:

When using 37% formaldehyde dosages are as follows for marine environments above 70°F/21°C (otherwise follow directions on the product’s label):

1) In-tank treatment: 1 milliliter (ml) per 10 gallons, or 2 drops per gallon daily in a bare aquarium.

2) 30-60 minute bath treatment: 0.6 ml per gallon, or 12 drops per gallon. Aerate vigorously and treat at this concentration for approximately 45 minutes. You can actually treat for a maximum of 60 minutes if the fish seems to be handling the bath fine, but cut it short (30 minutes) if breathing becomes too heavy.

These are other products which contain formaldehyde (e.g. Quick Cure, Aquarium Solutions Ich-X, Kordon Rid-Ich Plus), but because these contain less than 37% formaldehyde their effectiveness is difficult to predict.

Formalin is best administered in a bath solution for 30-45 minutes. The bath is best done in a large glass bowl or container, but a plastic bucket is fine as well. (However, keep in mind that plastic may absorb some of the medication and then leach it back out during future use.)

Temperature/pH/salinity of the bath water should match the tank the fish is coming from (and going into afterwards.) It is very important to heavily aerate the bath for at least 30 minutes before and also during treatment to compensate for oxygen depletion. The fish should be placed in a premixed bath solution. DO NOT add more formalin after the fish is already in the bath.

It is important to always transfer the fish into a new/sterile quarantine tank (QT) post bath. Returning the fish to the same tank you took him out of will just result in reinfection. It is also impossible to determine if just one formalin bath will result in 100% eradication of the pathogen you are trying to eliminate. Therefore, it is wise to do follow-up treatment in a QT:

1) Velvet: Copper or Chloroquine phosphate for 30 days.
2) Brook & Uronema: Metronidazole (e.g. Seachem Metroplex) or Chloroquine phosphate for 14 days.
3) Flukes & Black Ich: Prazipro or another formalin bath 5-7 days later.

It is also possible to achieve 100% eradication with repeat formalin baths, but it’s important to give the fish 48 hours to recuperate in-between baths. Post bath, always transfer the fish into a new/sterile QT to prevent reinfection. And observe closely for at least 30 days to be sure the fish is 100% disease-free.

You can dose formalin directly into a quarantine tank, but this can be risky due to the harshness of formalin and how quickly it can deplete the water of oxygen. Providing additional gas exchange to the QT is a must! This can be accomplished by using an air stone or pointing a powerhead towards the surface of the water. (Anything that causes ripples at the surface.) A fish being treated must be monitored closely and should be removed if showing signs of distress – this applies when treating in QT or in the bath solution.

ProsTreats or provides temporary relief for a wide range of diseases. In some cases, formalin can “buy you more time” until a proper treatment can be done.

Cons/Side EffectsFormaldehyde is a known carcinogen. Therefore, precautions must be taken when using it. Preventing your skin from coming in contact with it by wearing waterproof gloves and not breathing in any fumes by wearing a face mask is highly recommended. Formalin also can be harsh on the fish’s gills and will quickly deplete oxygen from the water. DO NOT USE if a fish has open wounds. There is anecdotal evidence (but never scientifically proven) that some fish exposed to formalin don’t live past 18-24 months. In some areas, the purchase of formalin is prohibited.

Further reading on using formalin to control fish parasites: