Used by itself or more commonly with marine fishes, in concert with formalin, Malachite Green is a powerful, though toxic medication with anti-protozoal and anti-microbial properties. A few cautionary remarks should always be associated with its use: To carefully measure the ACTUAL gallonage of the treatment system, as overdosing with Malachite is very easy to do... To take care not to spill or splash this material around as it is the dickens to get the bluish stains out of clothing, skin... or aquarium silicone. To use with caution period, as Malachite Green is toxic... poisoning your livestock, the "good guy" bacteria populations in your system, as well as (hopefully) the pathogens you hope to be rid of.
Malachite Green may well be the most ubiquitous compound found in fish "medicines"... It is used by itself, as well as in concert with other materials in the treatment of various biological diseases in freshwater, brackish, marine and pond applications. Though dangerous to use it (outlawed by some countries, e.g. the U.S., for use with food fish culture), Malachite Green continues to be a treatment of choice for mold diseases of fishes and their eggs and protozoan parasites. A few of the many manifestations/products consisting of, or partly made of this diarylmethane dye are listed above.
Malachite Green is used for anti-fungal purposes, in attempts at curtailing bacterial complaints, as an anti-protozoal (for freshwater ich and velvet mainly, but also monogenetic trematodes/flukes. It is used at times for other protozoan freshwater pathogens as well as marine ich/Cryptocaryon. The last is better treated with copper compounds in my opinion. Much easier to monitor, measure (there are no test kits for Malachite Green), and maintain physiological doses. Malachite Green is deadly toxic to all marine and freshwater invertebrates, algae, plant-life.
Re making your own... only if you are using HUGE volumes of water... like in a large fish hatchery/facility. There is a mineral "Malachite" which is not the malachite you're interested in, this is an oxalate or zinc chloride salt that must be zinc-free (otherwise it is deadly toxic to fishes). In fact, as dye lots vary tremendously I will dispense with actual formulation notes here. Almost anyone who is using Malachite Green for petfish uses will find it simple and cost-effective to purchase pre-made commercial solutions (these are almost always 3.7mg/ml.).
For petfish use, folks mainly use Malachite Green for short dips, or as prolonged treatments in recirculated systems.
For dips, up to 16 ml. of stock solution can be added to a liter (though this is the extreme highest concentration) and the fish left there, in a net, for ten to thirty seconds.
For added to the tank use 0.10 mg, or 0.10 ml. per gallon of treated water (do measure for the real gallonage of your system to avoid over-treatment) is added three times in three day intervals (nine days total). Excess Malachite is easily removed with activated carbon use.
Malachite Green's toxicity is temperature and pH dependent, being more toxic with rising temperature and lower pH. One surprising item re Malachite use is its inactivation (oxidation) with exposure to light. Malachite Green treated systems should have their lights turned off. I have rarely seen this mentioned with product labels or inserts.
Malachite Green should be utilized in a separate treatment tank as it is quickly absorbed by detritus, natural gravels... most all plastics...
Unfortunately, Malachite Green is suspected of being a carcinogen as well as teratogen (a chemical that can alter DNA make-up), and respiratory poison. It is NOT to be used lightly. This material is also phytotoxic... poisonous to aquatic plants and terrestrial ones that may come in contact with it from splash, spray or your dumping aquarium water on them from your treated tank.
Fish eggs that are close to hatching should not be exposed to Malachite... actually it's suggested to not prophylactically use this material on batches off eggs unless there is evidence of fungal infection. Small fishes, both marine and freshwater are sensitive to Malachite exposure. Fine scaled or naked fishes (small tetras, catfishes, knifefishes, mormyrids....) should only be administered with half doses... and carefully observed while under treatment for signs of poisoning.
So... is the use of Malachite Green "worth it"... That is, worth the risk of poisoning your livestock... A qualified "yes", particularly for freshwater and pond use... for fungused eggs for production facilities of these fishes, common ich and velvet problems... But not warranted IMO in marine applications... Just too dangerous IMO, easy to make mistakes with with seawater use. Better to stick with tried and true copper formulations that can be tested for, and have a bit wider efficacy.
Andrews, Chris, Exell, Adrian and Neville Carrington. 1988. The Manual of Fish Health. Tetra Press, NJ. 208 pp.
Herwig, Nelson. 1979. Handbook of Drugs and Chemicals Used in the Treatment of Fish Diseases. Charles Thomas, Illinois. 272 pp.
Noga, Edward. 1996. Fish Disease. Diagnosis and Treatment. Mosby-Year Book, Missouri. 367 pp.