Malachite Green for treatment of external fungal and bacterial infections in both freshwater and saltwater fish. Slimy Skin Disease, White spot etc. It would be beneficial to do a 25% water change before commencing treatment. After removing active carbon from the aquarium, add 1ml of Malachite Green solution per 40 litres of water, treat every other day for a total of 3 days. Change up to 50% of the water on the afternoons of the days that the treatment is not administered.