[michika]

Yellow tangs, in my experience, do exhibit a variation with more orange-yellow and butter-yellow, and that is on top of their ability to change colours while sleeping.

I have two yellow tangs that act and look slightly different from the others; they become darker when stressed or sleeping while my other yellows become pale with white spot which seems to center on their lateral line.

Feeding krill can bring out the reds and oranges in your fish and further accentuate complementary colours. It might not be a sign of illness, but one of good health and prosperity. Id still check out the link above and or the references here on Canreef just to make sure though.

**edit**

Sorry I couldnt see your initial photo at first. Yeah, there is something going on with your yellow tang. Check out the link here in the thread. You should probably get your tang into QT.