Fishman, I was just going to ask the same thing. We have only had our yellow tang for about 3 weeks and have noticed the same brown oval right under the white line at night. My thought, and only because I am a nurse and was thinking of the human body, is since they are resting and not moving there circulation is slowing and we are seeing that through their very thin skin. I could and am probably way off base about this. Anyone no the reason for sure????