Yellow tang help, please!

[Mikey777]

I've noticed around one week ago some areas of discolouration on my yellow tang's skin, which later developed into badly looking open wound...
is it something i can take care of and help to cure it?
hopefully picture will be handy for diagnosing.
it didnt loose appetite, other fishes don't show any symptoms of any disease.
no new species in my tank for months, so it couldn't be a problem.
this fish was living in my tank for 5 months without any problems, now it seems to be worsening condition day after day...

looking forward for your advices.

 

[Ohiomom]

First off, this tang is really, really skinny..not sure what you are feeding it. What size tank is it in? Parameters? Overall looks like your tang is suffering from environmental conditions and malnurishment..look up HLLE and it will give you some insight..there is no way that has happened overnight...sorry..not trying to be nasty but that is reality.