Ichthyophonus or better known as whirling disease
Obvious symptoms can be seen with the fungi causing paling of the skin and development of ulcers. Infected fish will usually swim in an erratic manner and usually it will cause internal organ damage with development of white colored cysts. Treatment is made even more difficult because of the nature of internal infection and usually bath treatment with short dip in freshwater is ineffective to counter the disease and fish usually die months after infestation.
Marine velvet disease
Similar like freshwater ich infection, marine velvet disease will cause the fish to be scratching against objects and the skin area will become dull with development of visible white spots. Treatment method involves short dip about 3 to 5 minutes in hospital tank filled with freshwater mixed with methylene blue solution added to concentration of about 3mg/liter. Same procedure can be repeated once every 3 days until the fish recovers. In fact, similar form of treatment applies to marine white spot disease as well.
Related: Basic fish medication and How to deal with sick fish
Reference: Stoskopf, M.K., Fish Medicine, W.B. Saunders Co., 1993