Malachite green (also aniline green, basic green 4, diamond green B, or victoria green B) is a toxic chemical used to treat parasites, fungal infections, and bacterial infections in fish and fish eggs. It is especially active against the fungus Saprolegnia, which infects fish eggs in commercial aquaculture. It is also a very popular treatment against Ich in freshwater aquaria.
The use of this substance has been banned in many countries due to its carcinogenicity in humans, although fish farms in many countries continue to use the substance.
Malachite green is used as a biological stain for microscopic analysis of cell and tissue samples. In the Gimenez staining method, basic fuchsin stains bacteria red or magenta, and malachite green is used as a blue-green counterstain. Malachite green can also be used to directly stain spores within cells; here a safranin counterstain is often used.
Malachite green is also used to dye materials like silk, leather, and paper. The chemical known as malachite green does not actually contain the mineral malachite — the name comes from the similarity of color.
