Koi, Goldfish & Pond Health In Twenty StepsDr Erik Johnson on Youtube with Koi Pond Fish Health Now on Amazon.com for Overnight Delivery


Keywords: Formalin Formaldehyde FMG Formalin Malachite Green Dosing for Koi Diseases - Parasites

Formalin can be available in gallon jugs for about sixty dollars.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.
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 preciptate - this is the toxic paraformaldehyde.

Use 2 cc 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 the treated system, 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:

This book has pages and pages on this subject and the book requires no electricity!

Notes From Doc Johnson
"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

If the pond is very large and cannot be changed that fast, 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
You can still buy Formalin & Malachite Green in a "still legal" shipping material (non hazmat) here.


Under certain circumstances, Mardel Clout can be considered a superior alternative to Formalin because it does not consumer Dissolved Oxygen, has no deleterious effect on fish, and minimal effect on plants. It is, however more expensive in larger facilities.

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