We were not feeding this fish enough!
Our yellow tang was looking a little thin and not swimming as actively. We were feeding this fish with a bunch of damsels in the water column with pellets and ground-up algae sheets. The tang was taking both out of the open water.
We know this type of fish will pick all day so we started placing algae sheets attached to a magnet in the tank. Our tang is feeding much more vigorously, is now more active, and has regained body mass.
How much do these fish feed in the wild? How much should they be fed in captivity? Who knows?
At this time we are feeding about 2 square inches twice a day. The damsels pick a little and the tang still takes a few pellets out of the open water during the twice day pellet feedings.