Yesterday there was a fairly large, pale spot on the bottom of one side of my sailfin tang. It was probably 1/2 inch by 1 inch in size and covered part of the lower body and most of the bottom fin, but only on one side. I did not see any sign of damage to the fin or the body of the fish, and the spot was gone this morning. The fish does change colors, usually just darkens and lightens, but it's throughout the entire body. Maybe his lower body and fin were in a different mood than the rest of his body. The fish is about 1 year old.