Fins wearing away sounds like fin rot, I'd recommend putting the fish in a hospital tank. The white on it's sides sounds like a fungus. Try some sort of fungus cure whilst the fish is recovering in the hospital tank.
Finrot and the body fungus can both be treated with an anti-fungal medication. No clue exactly how long it'll take, but the medication should tell you.
How many gallons is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
If the white on the fish is fluffy looking it columnaris.