We bought a Blue Tang today & when I got it home & under our tank light I noticed part of his black part in its back looks lighter almost faded. First time with a Blue Tang & don't know if this is normal or a sign it's sick. I have it in quanitine to be safe. I see no white spots on it so I don't think it's ick. It just looks faded in the area. Any ideas? Thank you in advance.