Malachite green is an effective treatment for a variety of fish diseases, including fungal infections, bacterial infections, and parasites. It can be used as both a preventative measure and a cure. Malachite green is safe for most fish when used as directed, but it can be toxic to some species. If you are unsure whether or not your fish can tolerate malachite green, please consult a qualified aquarium veterinarian or fish pathologist.