My Sailfin (Zebrasoma Veliferum) displays a 'fading' behavior right after he gets done eating a meal aside from his algea, nori, and veggies, mainly whatever I am feeding the other fish that he likes. This is his signal that he is full. I don't know if this is common in all sailfins, or tangs for that matter, but his behavior is very predictable. Within a 1/2 hour he is back to normal cooor and picking at the rocks. His behavior, although a little odd, does not seem to have any adverse effects. Just thought I would share.
MCF