Malachite green or MG is a vivid green chemical used commonly in our hobby to treat many diseases in freshwater and marine fish. It also has many other uses, including as a dye for fabric, leather, and paper.

While malachite in water is very effective at combatting fish disease, it is also highly toxic and banned in many countries for use on fish for human consumption.

This article will help you understand how to safely use malachite green for ich and many other fish illnesses.

What Is Malachite Green

Malachite green, sometimes called aniline or china green, is a chloride salt that is manufactured from organic compounds. Contrary to common opinion, malachite green does not contain any copper. The malachite green structure is complicated and not relevant to its use on fish disease.

The compound has been in use since the late 1800s, primarily as a dye for paper and leather. However, no one is certain when it was first used as an anti-fungal in fish keeping. It has similar properties as Methylene Blue but is stronger and harsher.

Malachite green is often mixed with formalin. The combination is a powerful medicine useful against a long list of fish fungi and parasites including:

  • Ambiphyra – A common gill and skin parasite that can respond well to a single treatment.
  • Chilodonella – Manifests as cloudy mucus on the skin of the fish. They are usually cleared with a single treatment.
  • Costia – Sometimes called blue slime disease because of the secretions that form on fish. Usually easily treated.
  • Cryptocaryon (marine Ich) – The various forms of Ich are more difficult to treat and usually require repeated cycles of treatment to kill both the infection and unhatched cysts. If one fish is seen with Ich, all fish in, the aquarium should be treated.
  • Dactylogyrus – Gill flukes
  • Epistylis – Generally not a problem unless water quality deteriorates. They are easily treated.
  • Gyrodactylus – Skin flukes
  • Ichthyophthinus – Freshwater Ich
  • Oodinium – Also called fish velvet, affects salt and freshwater fish. They are easily treated.
  • Trichodina – A common parasitic infection that is usually cleared with one treatment.

Safe Handling

Today malachite green is most often purchased as a premade compound. For best use, you should follow the manufacturer’s directions for mixing and dilution. Malachite green is a powerful chemical, and too much can kill fish eggs, and fish.

It is also a toxic substance and can be harmful to humans, enough to be banned for use on fish eggs and fish intended for human consumption.

There are some rules to follow to ensure safe use for yourself and your fish:

  • Wear gloves when handling.
  • Wear eye protection when handling.
  • When mixing Malachite green with water in a container, do not use that container for anything that might be consumed by humans or other pets afterward.
  • Malachite green is a respiratory poison, so do not breathe or inhale fumes. Wearing a mask is recommended.
  • Do not store in a clear container. Malachite green is deactivated by light.
  • Do not use with species such as catfish, loaches, invertebrates, or tetras. It can be fatal to them.
  • Follow the manufacturer’s directions for mixing, use, and storage.

Preparing Malachite Green

Malachite green is most often sold as a concentrate in various sizes of containers. When mixing, the ratio will vary depending on the manufacturer, treatment type, and fish size. The common range of malachite green to water will average 0.05 to 0.25 ppm.

Concentrated malachite green can be pre-mixed with distilled water and stored in a dark container for it to last long. The solution can be easily added to a quarantine or treatment aquarium when needed.

Treating Fish Fungus

Malachite green fish treatment is placing fish in an aquarium holding a solution of fresh or saltwater and malachite green.

Treatment is best done in a separate quarantine aquarium so that the chemical does not harm invertebrates or other species that may be in the home tank.

The quarantine aquarium does not have to be as large as the home tank, nor it needs to be fully decorated. It simply has to be large enough to hold the ill species.

Set up the quarantine tank with water conditioned to the needs of the species to be treated. Make sure that the water temperature matches the home tank and that the quarantine aquarium has fresh filter media. Do not use carbon filter media as it will scrub much of the healing malachite from the water.

Once you have added malachite green in the aquarium according to the label instructions, you can transfer the fish needing treatment to the tank. There are two basic types of treatment. These are dip and prolonged exposure.

Dip treatment is exposing infected fish to a stronger solution of malachite green for a short duration. Exposure times are usually 30 to 60 minutes.

Once the fish have been dipped, they can be transferred back to their home tank or placed in a clean water quarantine tank. Observe them over the next 24 hours.

If the infection was light or superficial, fish would often respond favorably to a single treatment. If the infection is more severe, it may take prolonged exposure treatments to cure.

Prolonged exposure treatment is keeping fish in a milder solution of malachite green for a period ranging from 24 hours [1 cycle] to 36 hours [3 cycles].

In prolonged exposures, the fish are observed for 24 hours. If additional treatment is needed, make a 25% water change, and add malachite green at the specified mix to the tank volume. This is considered one cycle.

Trying to treat fish over more than three cycles can expose your fish to lethal doses of malachite green.

Note that sometimes treating the fish will not treat the cause of the infections if they are caused by conditions in the home tank.

If you are treating fish in a quarantine tank, also give the home aquarium a thorough cleaning, new or newly cleaned filter media, and up to a 50% water change. This will help ensure that your treated fish do not get sick again upon return to the home aquarium.

Treating Fish Eggs

Malachite green can be an effective solution for treating fungal infections on fish eggs. While this is strongly not recommended for use on eggs that will hatch info fish for human consumption, it is a viable alternative for tropical or ornamental fish breeders who wish to preserve clutches of eggs.

Fish eggs are fragile and are treated differently than live fish. In general, the treatments are of shorter duration and are of a dip or bath type.

Fish eggs are usually collected before treatment by gently netting them or taking the structure they are adhered to, such as a plant stalk or moss patch.

In dip applications, the eggs are placed in the dip bath for a very short duration, anywhere from 10 to 30 seconds, and then placed back in the home aquarium for observation. If the infection disappears over the next 24 hours, the eggs have a good chance of surviving to hatch.

If the infection does not clear up, treatment can be repeated, but excessive handling can damage the eggs.

In bath-type treatments, the malachite green is added to the aquarium holding the eggs. After 24 hours the filter media is replaced or cleaned, and a 25% water change is done.

The filter will remove the malachite green from the water. If the infection is not cleared after an additional 24 hours, the process can be repeated.

If the infection does not clear after a few cycles the clutch is most likely compromised, and it will be a good idea to thoroughly clean the aquarium and start over.

One note on bath-type treatments. They will kill any invertebrates you have in the tank, so if you do keep snails or shrimp, do not treat the water in that home aquarium.


  • Malachite green is an effective chemical treatment for fungal fish diseases and infections.
  • The compound does not contain copper as many hobbyists believe.
  • Malachite green is highly toxic. Wear proper personal protective equipment when handling.
  • Commercially available malachite green will vary in level of concentration. Follow the manufacturer’s guidance for proper mixing and use.
  • Malachite green should not be used on fish or fish eggs intended for human consumption.
  • The compound will kill invertebrates such as snails and shrimp. Do not use it in the home tank if you keep these species.
  • Dips are short-term exposures to higher concentrations of malachite green.
  • Baths are prolonged exposures to lower concentrations of the compound.

If you have fishes with fungal infections, malachite green is an effective medicine to combat those illnesses. Follow the manufacturer’s directions for mixing and use, and you will help maintain your aquariums with healthy fish and habitats.

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