Ichthyopathogenic unicellular organisms are the cause for many fish diseases. Among them,, the white spot disease pathogen Ichthyophthirius multifiliis is certainly the most important one. Almost every aquarist has come into unpleasant contact with this dangerous pathogenic ciliate. The parasites infect the whole fish stock in the aquarium within shortest time. As these single-celled organisms mainly parasitise on the fish skin, young fish or those that have only small scales are particularly in danger. The active agent in Protazol, Bis(4-dimethylamino-phenyl) phenyl methylium hydroxide (malachite green base), is highly effective against swarmer stages of the parasite. At the same time it is tolerated well by the fish. Besides Ichthyophthirius multifiliis, Protazol is also effective against other ichthyopathogenic ciliates such as Trichodina sp. and Chilodonella sp. Note: In case of an infestation with Ichthyophthirius multifiliis, raising the aquarium water temperature to 30 ŚC during treatment in several steps will increase chances for a successful treatment.