what kind of fish? and what kind of fish do you have in your tank? not all kinds of fish can tolerate salt especialy catfish or scaleles fish. Plectos hate salt. And aquarium salt for fresh water is only high priced kosher salt uniodized. Most tetras and Amazonia fish don't tolerate salt either. Salt added to fresh water should be the same salt that you add to saltwater tanks. Saltwater fish drink water fresh water fish don't they obsorb. There are a lot of fungus cure meds. If you don't know what your dealing with you should use a broad spectrum antibiodic. You didn't even give a description of the disease or what part of the fish its on. Cant answer your question with out knowing anything!
