Malachite green
Basic triphenylmethane dye. Effective against many parasites, fungi, protozoa, ciliates. Usually used as oxalate but the active ingredient is better tolerated as chloride. However, as the oxalate is rather poorly soluble in water, it is favoured for synthesis because it is easier to isolate and purify.
The best known and probably most potent active ingredient against the plague spot disease.
Caution: The active ingredient is classified as potentially carcinogenic !