Malachite Green

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The silicone in a 10 gallon tank a day after adding only 3 drops of a 1% malachite green solution, which used to be fully white before the treatment. The lower half of the image has a black overlay highlighting the change in color.

Malachite green (MG) is a synthetic dye used to colour silk, wool, jute, leather, cotton and paper. The name comes from the similarity of colour of malachite green to the mineral, malachite.

Malachite green has traditionally been used to treat fungal infections on fish and their eggs.

It is a potent chemical dye and is often used as a 1% solution (typically use 1g of powder dissolved in 100ml of distilled water) and then dapped directly onto the infected area of the fish with a cotton bud daily until the infection is healed. Or a bath is made and the fish dipped into it for a time.

  • Use a solution of half-potency (0.5%) if treating tetras or small fish as these are sensitive to this dye.
  • This dye's effectiveness is reduced by light, therefore during treatment turn the tank lights off.
  • Note: This chemical is harmful to humans!

Various ornamental fish treatments use this dye. It will probably be banned from the ornamental fish hobby in the near future due to concerns of its risk to humans.

Malachite Green and Formalin are two of the key chemicals for treating fish disease and have been used for many years against a range of parasites. They can be used together or separately as anti-parasite treatments against as Gyrodactylus (skin flukes), Dactylogyrus (gill flukes), Ichthyobodo (Costia), Trichodina , Chilodonella and Ichthyophthirius (white spot).

Malachite green also has powerful anti-fungal properties and is used against Saprolegnia (fungus) either when present on fish or to as a treatment to protect fish eggs from infection.

Farm use[edit]

Malachite green has been used extensively by the aquaculture industries in Europe and throughout the world for many years in the absence of any authorised veterinary medicine alternative. It has proved particularly effective at protecting the welfare of farmed fish. The UK Government has taken scientific advice from independent advisory committees on the use of this product to ensure that the interests and health of consumers are protected. However, there are continuing concerns about its potential effect on human health.

A veterinary medicine alternative to malachite green called "Pyceze" has now been developed in the UK with the assistance of funding from the salmon and trout industries and DEFRA. The active ingredient of this new product, "bronopol," was permitted by the EU for use in the treatment of fish for the first time in 2002. The Veterinary Medicines Directorate has granted a provisional marketing authorisation for Pyceze and they are currently assessing the suitability of the product for full marketing authorisation. In the meantime Pyceze is available for the treatment of fish and their ova under veterinary prescription.