ok this is from google
METHYLENE BLUE is effective for the control of superficial fungal and bacterial infections of freshwater and marine fishes. As a secondary use, it is effective against various external protozoans, including ichthyophthirius, Chilodonella, Ichthyobodo (Costia) and Epistylis. METHYLENE BLUE is also useful for the treatment of known nitrite and/or cyanide poisoning of aquarium fishes. This drug aids in the transport of oxygen and converts abnormal methemoglobin back to normal oxygen carrying component, hemoglobin.