Cotton Wool: Fish Are Covered in White Fungus
Fungus is nearly always a secondary infection, so it is important to find out the underlying cause, which could be water quality, parasites or a bacterial infection. Obviously these need to be resolved at the same time as treating the fungus. If the affected area is fairly small a combination of topical treatment and a long-term bath in malachite green or some other anti-fungal treatment should be successful. However, if the affected are is large, the outlook is poor.
Youtube Video About Fungal Infections