Some medications use a compound called formalin in them. Formalin is simply a 30% solution of formaldehyde, the stuff which preserves frogs for dissection.
Believe it or not, this compound supposedly kills all the following pathogens;
- protozoan parasites
In actuality this is true. At a high enough concentration formalin kills the exposed living cells of all these organisms. Unfortunately, at that concentration, it also kills the cells in the gills of the fish. Whoops!
Research on formalin to treat ich in catfish (“Treatments for Ich Infestations in Channel Catfish Evaluated under Static and Flow-Through Water Conditions,” Tieman et. al. 2001) showed it did little damage to the ich theronts at 25 ppm, a lot of damage to the ich theronts at 50 ppm and it killed all the catfish at 100 ppm. This is a narrow band of treatment. One should never just “add a little more” formalin medication.
One interesting myth about formalin repeated over and over again is that formalin which has a white precipitate in it is deadly poison to fish. Formaldehyde can polymerize into polyformaldehyde (otherwise known as acetal plastic or “Delrin”). This happens if the formalin drops below 50 degrees. This white material is innocuous and won’t harm anything. It also removes the formaldehyde, so it stops the medication from being effective. But it quite decidedly is not poisonous.
The University of Florida found formalin to be effective against ich, so it does work against ich (also velvet, costia, chilodonella and epistylis). Formalin is what public aquariums use as their ich medication. Several other studies showed formalin/malachite green to be effective against ich. Note that formalin is short lived in the aquarium, so treatment should be every day.
I use Ich X when I get ich, as does Cory and Joey. Ich X contains formalin. I’ve never killed any fish with it so it is safe to use. But do not overdose it. That could be deadly. Also do not combine raising temperature with formalin. That also can be deadly.
Formalin/malachite green is also very effective to prevent saprolegnia (“fungus”) in fish eggs. All eggs should be treated with this at the same strength used in ich treatments to prevent the eggs from “going bad”.
Weak solutions of formalin are best described as weak antiseptics. Antiseptics have many shortcomings. Antiseptics are examined in more depth in this link: