Pond Use of Formalin for Koi Diseases

Formalin is a mixture of formaldehyde gas in water. 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.

Pond Use of Formalin for Koi Diseases

Pond Use of Formalin for Koi Diseases
Pond Use of Formalin for Koi Diseases

To use this, try to get Methanol free, but if not, expect some trouble from your smaller treated fish. Never use Formalin that contains a white precipitate – this is the toxic paraformaldehyde.
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.

  1. Formalin will kill weak or piping fish.
  2. Formalin will kill filter bacteria.
  3. 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
“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 Pond Use of Formalin for Koi Diseases

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
What if Formalin is not available anymore with the new 911 shipping restrictions?
Proform C is perhaps your best choice as far as Formalins go. Part of the reason for this is that since 9-11 the shipping industry and the Federal government have tightened shipping restrictions which make pure Formalin illegal to ship without stringent Hazmat labelling and fees. A gallon of science-grade Formalin which once cost $16 now costs $64 and requires a ream of paperwork.
Proform C is our saviour as more and more Formalin Malachite compounds get sidelined by hazmat shipping problems. Proform C can be used on almost every parasite of Koi, but it’s specialty is protozoans including Chilodonella, Costia!!, Trichodina, more!
Proform C is great, and I’ve used it with good effect. It’s readily available and easy and legal for shipping. Somehow, it also seems to keep Formalin affordable. Since Proform C is not a proprietary 37% Formalin solution, you should use and follow the guidelines which are on it’s label or which ship with it.

Hint: I’ve used it at 50% over the specified dose with good results.


Dr Erik Johnson is a Marietta, Georgia Veterinarian with a practice in small animal medicine. He graduated from University of Georgia with his Doctorate in 1991. Dr Johnson is the author of several texts on Koi and Pond Fish Health and Disease as well as numerous articles on dog and cat health topics.