Formalin + Malachite Green (FMG)
A long-standing parasite treatment with real strengths and real limits. Powerful when used correctly. Easy to misuse when temperature and oxygen are ignored.
What FMG is
Formalin + Malachite Green (FMG) is a combination treatment used for decades in koi ponds, hatcheries, fish farms, and research facilities. Formalin is an aqueous solution of formaldehyde. Malachite green is a synthetic dye with antiparasitic properties.
Used together, they form a synergistic external parasite treatment that can be highly effective when used correctly — and unforgiving when used carelessly.
What it’s effective against
- Costia
- Chilodonella
- Ich (Ichthyophthirius multifiliis)
- Oodinium
FMG works by direct contact. It is not a bacterial cure, not a fluke treatment, and not a long-term pond medication.
Temperature matters
Warm water: smaller margin for error
In warm water, fish oxygen demand rises while formalin consumes dissolved oxygen. Stress tolerance narrows. Even “normal” dosing can become dangerous when aeration is marginal, organic load is high, or the pond is heavily stocked.
Cold water: slower recovery and delayed problems
In colder water, FMG can still work, but recovery and detoxification slow down. Problems tend to show up later — prolonged stress, delayed recovery, and secondary issues days after treatment rather than immediate kills.
Oxygen demand and aeration
Formalin consumes oxygen as it reacts in the water. The effect increases with temperature, organic load, and stocking density.
Rule of thumb
If you run FMG without aggressive aeration, you’re gambling. Most “mystery losses” blamed on FMG are oxygen problems.
Using FMG with salt
Salt does not automatically make FMG unsafe. Many experienced keepers have used FMG in salted ponds without immediate issues. The concern is stacked physiological stress and reduced tolerance for mistakes.
What changes with salt
Salt alters osmotic balance and gill function. FMG stresses gills and oxygen exchange. Combined, the margin for error becomes smaller. Effects can be delayed: slower recovery, increased susceptibility to secondary infection, and problems days later instead of during dosing.
Storage, paraformaldehyde, and cold damage
Formalin can polymerize into paraformaldehyde after cold exposure or poor storage. Cloudiness, white flakes/crystals, or sediment are signs a product should not be used.
Staining
Malachite green is literally a dye. It can stain skin, clothing, liners, and porous surfaces.
Proven formulations
FMG has been used long enough that stable, predictable formulations exist. Aqua Meds Terminate is an example of an FMG blend refined through years of real-world pond and fish farm use, emphasizing stability and predictable behavior.
FMG is effective — and unforgiving. Respect oxygen, temperature, and storage quality.