Purple tang with HLLE help and advice

[way2addicting]

First off I'm not interested in having the tang police saying I have to small of a tank, yes I know a 65 is to small however due to the circumstances I have a purple tang!!! I have looked up HLLE info and know it can be from a ton of different things but haven't seen any kind of success stories and I'm wondering if trying to save this tang is at all possible? My lfs has had him for a few months he came from a tank where water quality was horrible and fish were neglected and he has been in a 20 gallon holding tank. He has no top fin or bottom fin with very bad lateral line and head erosion (I'm guessing will post pics when I get home) and is almost completely white from stress I assume but tail fin is yellow. No one has been interested in him and they were going to get rid of him so i got him to try and help him out and give him a better life than the shitty one he's had so far. The first day in my tank he hid in a cave all day only came out a few times just to make a lap and back to the cave, today he has color and looks purple which he wasn't every time i saw him at the lfs. I also have nori in there and the second day when he started venturing out he found it and ate the whole piece I put in for him so u put in another and it was gone, the 5th sheet he finally started picking at it and not just eating it all. So I'm guessing his diet wasn't what it should of been there. He goes through about 3-4 pieces a day now has colored up even more and is now swimming alot. He isnt as skidish around me now so I know he has made alot of progress in my tank in the 3 days from the 2 moths I've seen him at the lfs. But will his face ever heal and the lateral line and will his fins grow back? I don't care if it's gonna take awhile at this point the 3 days I've had him and his big improvements I know he's happier even if it is a 65 gallon and now I just wanna get him better