Malachite green and methylene blue are very old remedies which were put forth in the 30’s and 40’s as fish medications. They are aniline dyes. They have a well documented ability to selectively control ich, the most common tropical fish disease. They are also very mild antiseptics which are useful for killing saprolegnia (“fungus”) and they kill some types of bacteria in the water column.
None of these dyes are used or ever have been used to treat any human or pet pathogens. None of them have every even been used as antiseptics, like the compounds hydrogen peroxide, iodine, mercurochrome, iodoform and merthiolate. No one to my knowledge has ever used methylene blue to sterilize a wound. Methylene blue is so harmless it is used as a joke during fraternity pledging. Take a tablet of methylene blue and you pee blue for several days (personal experience). Methylene blue is ONLY used in humans with some forms of poisoning where the blood becomes compromised.
Malachite green is the dye most prevalent in commercial aquarium medications. Quoting the one site which sells malachite green:
“Malachite green is a well-known and old remedy that has been used to successfully treat not only ich but ectoparasites (gill flukes) as well as bacterial and fungal infections”.
There is an old saying in the medical field, “when a medicine claims to cure just about everything, in fact it cures nothing”. Ich, flukes, bacteria, fungus! Wow! It’s a miracle cure! These broad claims are just impossible. Malachite green will stop an ich outbreak (confirmed by university research). It kills some bacteria in the water column but is of no use in bacterial infections in fish such fin rot. And, like many disinfectants, it will kill saprolegnia (“fungus”).
If you want a real miracle cure just look at the claims for methylene blue. It supposedly “cures nitrite poisoning, ammonia poisoning, ich, velvet, swim bladder disorder, fungus and fish stress”. “Fish Stress”? Is that laying it on thick or what. This dye selectively kills ich (proven by university research) and probably does do some some disinfecting of eggs and some prevention of disease transmission in a quarantine tank. Its structure has many parallels to the structure of malachite green and it can probably be used interchangeably with malachite green.
Indeed, one type of ich “cure” (“FMC solution”) was the combination of formalin with both malachite green and methylene blue. This would probably work as well as the formalin/malachite green combination.
Acriflavine is another aniline die used in fish medications. It is just slightly less of a miracle cure, supposedly stopping velvet, cotton-mouth disease, fin rot, fungus and bacterial infections. The research on this dye says it doesn’t work, period.
Methylene blue is unique in that it seems to kill some types of bacteria in the water column while not harming the gills of the fish. Methylene blue is thus a safe “anti-bacterial”, “antifungal” that is somewhat useful in tanks of fish to prevent transmission of pathogens such as columnaris between fish. It is thus useful prophylactically in the water column in most quarantine tanks if there are several fish in the tank. But it doesn’t seem to kill anything if used in fish food. So it can’t be used for diseases which are already inside the fish.
And methylene blue and malachite green are both very effective treatments for eggs, keeping the saprolegnia organism (“fungus”) off the eggs very effectively. But both methylene blue and malachite green can stain the silicone in the corners of a glass tank and are avoided by some hobbyists because of this.
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