KORDON Methylene Blue-General Disease Prevention Treatment for Aquarium, 4-Ounce [2-Pack] Methylene Blue is effective against superficial fungal infections of fishes. The drug may be used as an alternative to Malachite Green for the control of fungus when it is known that the fish to be treated are sensitive. Methylene Blue is safe for use with fish eggs and fry for the prevention of fungal infections. As a secondary use, it is effective against some external protozoans, such as Ich, Chilodonella and Costia. Treatment For Aquarium 4oz