Formalin Malachite

Formalin / Formaldehyde: How to use Formalin Malachite for Parasites

Formalin For Koi Diseases

Formalin Malachite
Formalin Malachite is a good treatment for parasites (and some funguses) in tanks and ponds.

Formalin – by Doc Johnson
Formalin is a mixture of formaldehyde “gas” in water that’s used to treat parasites in aquarium and pond fish.
It’s an odd chemical: It’s not an easy mix to stabilize and sometimes they use Methanol to make it more stable.
Formalin and Formaldehyde are essentially the same things. Formalin, by definition, is Formaldehyde at a concentration of 37% in water.
To use this, try to get “Methanol free”, but if not, expect some trouble from your smaller treated fish (because methanol is stressful at least for a minute while it degasses). Never use Formalin that contains a white precipitate – this is the toxic “paraformaldehyde”. Paraformaldehyde forms in Formalin when it’s older, lacks methanol or more often, gets chilled or frozen. Throw it out. Don’t pour off the Formalin and think it’s safe because I can tell you from experience, it’s not.
If you can do a water change: Use 2 cc of Formalin 37% per 10 gallons to create a 50 ppm concentration in the system, wait two hours and flush out the treatment water, sucking the outlet water from a bottom drain or vacuum if possible. If possible, remove the biological media to a fishless vat for the treatment period. Treat every day or every other day for three to five treatments for best results.

For plants, use 3 cc per 10 gallons for a mere 12 hours to eliminate pests that the plants may carry.

  • In main systems, formalin binds oxygen and so you need to aerate.
  • Formalin will kill weak or piping fish.
  • Formalin will kill filter bacteria.
  • Formalin will consume massive amounts of oxygen while in use.

More information, and specifics on three different ways to use Formalin with minimal side effect, as well as a regimen for the Salt Resistant Costia; are available at: Koi Health and Disease 2: A book I wrote and is selling on

“Don’t expose any fish to Formalin for more than one hundred and twenty minutes. Use half dose for Goldfish. That means you’d only use 25 PPM for one hundred and twenty minutes then do a massive water change.” ~ Doc Johnson

Note From Doc Johnson on Big Ponds:
If the pond is very large and cannot be changed that fast (120 minutes), drain the pond halfway, and treat at fifty PPM, then at the half-hour mark you’d begin refilling to dilute the Formalin as fast as you can.” ~ Doc Johnson

Proform C can be used on almost every parasite of Koi, but it’s specialty is protozoans including Chilodonella, Costia!!, Trichodina, more!