Treatment.

FMC: treatment or ritual?

FMC is one of those mixtures that has become so firmly established in aquarium practice that simply using it is sometimes regarded as sufficient action.

The fish is rubbing — FMC.
Mucus appeared — FMC.
Parasites are suspected — FMC.
It is not quite clear what is happening — FMC again.

The problem is not the mixture itself. Formalin, malachite green, and methylene blue do have biological activity and can be useful in certain situations Link: https://fossamethod.ru/ru/posts/medicines_poisons/ . The problem begins when FMC ceases to be a means of acting against particular groups of pathogens and turns into a universal procedure: “give the fish a bath”.

Where did FMC come from in the first place?

FMC did not originate as a ten-minute “bath for everything”. Gerald Bassleer’s handbook states that the precursor of the mixture was the Leteux–Meyer Mix, proposed in 1972: 3.3 g of malachite green per 1 L of formalin (Leteux & Meyer, 1972). In 1979, G. Bassleer proposed a stock solution containing three components (Bassleer, 2010):

3.5 g malachite green;
3.5 g methylene blue;
1 L formalin.

The recommended dosage is 1 ml per 100 L of water, with a single treatment intended to provide approximately two days of exposure. For ichthyophthiriosis, a water change followed by repeated treatment after two days was recommended (Bassleer, 2010). In other words, the original logic of FMC use was not a few minutes of contact with a concentrated mixture, but prolonged exposure to a comparatively low concentration of the preparation in water.

For comparison, the same handbook gives two fundamentally different treatment regimens for formalin alone (Bassleer, 2010):

1–2 ml per 100 L of water for 12–48 hours;
0.125–0.25 ml per 1 L of water for 30–60 minutes.

So even for the same substance, the treatment regimen is determined not only by the name of the drug, but by the combination of concentration and exposure time. This is an important point.

It is not enough simply to say: “Formalin is used against ectoparasites”.

The same formalin can be used as a prolonged treatment at a relatively low concentration or as a short bath at a much higher concentration. These are already different exposure regimens.

This is exactly why the phrase “Let’s bathe the fish in FMC for 10 minutes” tells us almost nothing by itself about whether the treatment is rational.

We need to know:

the concentration of the mixture;
the duration of exposure;
the suspected pathogen;
the sensitivity of the fish;
the water conditions;
and, most importantly, where this particular regimen came from in the first place.

If a source specifies tens of minutes or tens of hours for formalin, but multi-day exposure for FMC, then a ten-minute FMC bath requires its own justification.

The name of a drug does not determine the treatment regimen. The regimen is determined by dose, time, and purpose.

The fact that a fish was bathed does not mean anything was treated

The phrase “the fish was bathed in FMC” describes a procedure, but says almost nothing about treatment. To evaluate the result, we need to know at least:

what was suspected
what was found before treatment
what concentration was used
how long the exposure lasted
what water the bath was performed in
what changed afterward
whether the suspected pathogen remained

Without this information, FMC easily turns into action for the sake of action. We see that the fish was placed in a colored solution for ten minutes, and psychologically it creates the impression that treatment has been performed. But treatment is defined not by the fact of contact with a drug, but by whether the biological objective was achieved.

“If it got better, then it worked”

Not necessarily.

After a bath, a fish may look better for several reasons:

the amount of surface mucus decreased;
some ectoparasites actually died or detached;
the irritating load on the skin changed;
the fish was transferred into cleaner water;
for some time, the fish had less contact with the source of reinfection.

None of these possibilities by itself proves which component of the mixture had an effect or that the cause of the disease was eliminated. And an improvement in appearance certainly does not always mean that the pathogen has been destroyed.

You can test your reasoning here: https://fossamethod.ru/ru/test/reasoning-test/

Response to treatment is an observation. A diagnosis is already an interpretation of that observation.

They are not the same thing.

What if the parasite really is sensitive to FMC?

Then using the mixture may be entirely justified. But even here, questions remain.

At which stage of its life cycle is the parasite sensitive?
Is it on the surface or deeper in the epithelium?
Is the exposure time sufficient?

Could the therapeutic concentration also be too stressful for an already weakened fish? A short bath prescribed simply because “we usually use FMC” looks particularly questionable. If the exposure is insufficient for the treatment target, but sufficient to irritate the gills and skin, we get the worst of both worlds:

the parasite survived the treatment, while the fish received an additional burden.

This is no longer a question of whether FMC is a good drug or a bad one. It is a question of whether the method matches the task.

Why a mixture is convenient — and therefore dangerous

A mixture of several active components is psychologically perceived as more reliable:

“If one component does not work, another one will”.

But the same principle works in the opposite direction. Each component has its own toxicity, its own spectrum of activity, and its own limitations. Therefore, adding a second and third substance not only broadens the potential action of the mixture, but also increases the number of factors that must be taken into account.

The more complex the mixture, the less sense it makes to use it without understanding the cause of the disease.

The most inconvenient situation

Imagine this: a fish shows nonspecific behavior, for example rubbing against objects. It is bathed in FMC. A day later, the behavior decreases. What has been demonstrated? Practically nothing. We do not know:

whether there was a parasite at all;
which parasite it was;
whether it was on the skin or gills;
whether the mixture affected it;
which component of the mixture was significant;
whether the behavior disappeared for some other reason.

And yet this is exactly how traditions are born:

“FMC helps when the fish is scratching”. After some time, the original observation turns into a treatment rule.

Where are the “traditions instead of science” here?

Not in the fact that FMC is an old preparation. And not in the idea that it cannot be used. The problem begins when the name of the mixture replaces the definition of the task. “Do FMC” becomes a goal in itself. Although the correct order is the reverse:

determine what we are observing;
formulate a working hypothesis;
understand what target we want to act upon;
choose the drug and exposure regimen;
check the result.

If the first three steps are missing, treatment easily turns into a ritual.

A drug does not become scientifically justified simply because it has been used for many years.
But an old drug does not become bad simply because it is old.
There is always one question: why exactly are we using it here and now?

Literature

Bassleer, G., 2010. Новый иллюстрированный справочник болезней рыб для декоративных тропических и прудовых рыб: наблюдение, диагностика, профилактика, лечение. Пер. Н. О. Лисова. М.: Учебный центр “Аква Лого”. 232 с. ISBN 978-5-9902638-1-9.
Main source for the FMC formulation cited above, the note that G. Bassleer developed the stock solution in 1979, the dosage of 1 ml/100 L, and the formalin treatment regimens.
https://www.researchgate.net/publication/327941229_New_illustrated_guide_to_fish_diseases

Leteux, F., Meyer, F.P., 1972. Mixtures of malachite green and formalin for controlling Ichthyophthirius and other protozoan parasites of fish. The Progressive Fish-Culturist, 34(1), 21–26.
Classic paper on the malachite green–formalin mixture for treatment of ichthyophthiriosis and other protozoan infections; in later literature, this combination is often referred to as the Leteux–Meyer mixture.
https://doi.org/10.1577/1548-8640(1972)34%5B21:MOMGAF%5D2.0.CO;2