What It Treats: Temporary relief for Marine Velvet Disease (Amyloodinium), BrooklynellaUronemaFlukes (Monogeneans), Black Ich (Turbellarians). 100% eradication of these diseases is possible through repeated baths and/or In-tank treatment.

How To Treat: In order for Formalin to be fully effective, the product you are using should contain 37% formaldehyde + 10-15% methanol. There are very few “aquaculture approved” formalin products on the market, and they can be expensive + difficult to buy due to various regulations. Parasite-S (MSDS is here) is aquaculture approved and can be purchased from Syndel USA:


Or this one:

** Keep in mind that formalin typically only has a 6 month shelf life, so don’t stock up on it! :) Also, formalin should always be stored + transported at temperatures above 40°F (5°C).

Usage: The below dosages only apply to fish being treated in a marine environment above 70°F/21°C:

1) In-tank treatment: * 0.9 – 1 ml per 10 gallons daily in a bare aquarium for 7-10 days. This produces a concentration of ~ 25 ppm. Dosage may be cut in half (0.5 ml per 10 gallons or 12.5 ppm) for sensitive species or extremely sick fish that are unable to tolerate a full treatment.

* 0.9 ml per 10 gallons if dosing formalin WITH copper in the water
* 1 ml per 10 gallons if dosing formalin WITHOUT copper in the water

2) 30-60 minute bath treatment: 1 ml per gallon premixed into saltwater. This produces a concentration of 250 ppm. Dosage may be reduced to 200ppm (0.8 ml per gallon) or even 150ppm (0.6 ml per gallon) for sensitive species or extremely sick fish that are unable to tolerate a full treatment. Treat for a maximum of 45-60 minutes if the fish seems to be handling the bath fine, but cut it short (30 minutes) if breathing becomes too heavy. The bath is best done in a large glass bowl or container, but a food grade plastic bucket is fine as well.

Anytime you use formalin it is extremely important to vigorously aerate the water, especially when doing the bath treatment. This is because formalin has a tendency to rapidly reduce available oxygen in the water. You can use an airstone (or multiples) cranked on high, or a powerhead pointed towards the surface of the water, to increase gas exchange (which provides more oxygen).

ProsTreats or provides temporary relief for a wide range of diseases.

Cons/Side Effects: Formaldehyde is a known carcinogen. Therefore, precautions must be taken when using it. Prevent your skin from coming into contact with it by wearing waterproof gloves, and avoid breathing in any fumes by wearing a face mask. Formalin also can be harsh on the fish’s gills and will quickly deplete oxygen from the water. DO NOT USE formalin if a fish has red sores/open wounds. In some areas, the purchase of formalin is prohibited.