This would be so much easier with a picture of the tang and the affected area.
I know.....I wish I had a digital camera. Ill see if i can get my hands on one! It doesn't look like the picture you have posted. In that picture, the infected areas look "indented". The marks on my fish do not look raised or indented...its as if the yellow color is mising....its hard to explain. I spoke with someone at the LFS
and he said it was common with the tangs and he has had the same problem with his before and he did the fresh water dip.
I have done the half and half dip with the formalin twice now.....he is much more active today and I think his eye is clearing up a little bit! The discoloration is still present though. Oh yea...he seems very shy, anytime someone walks by the tank, he hides in his cove...is this normal or does he just not like me?!