Hi, Nanomania ... and with condolences for your loss of a lovely tang. Humblefish is probably right - there doesn't seem to be a mark on the fish, save those that might occur as a fish's life leaves, and early-stage decay begins to set in. The fish isn't obese, but he's not emaciated, either. I'd say the fish had decent weight. Sometimes we can put a finger on why a fish died, and all we can do is speculate. Cyanide? Heart attack? Stroke? Nighttime fright causing the fish to crash into a rock? Internal parasites? We'll probably never know, without a pathological lab necropsy.
In every respect, your "Philippine Yellow Tang" looks like Acanthurus pyroferus, more commonly called the mimic tang or chocolate tang here in the U.S. It looks as though his fins were beginning to elongate, and I would have expected to see the brown and olive colors of adulthood creep in. A. pyroferus is sometimes sold as "Philippine yellow tang", so that ID would make sense. Juveniles mimic lemonpeel angelfish; adults, while less brilliantly colored, are elegant and refined.
~Bruce
