You don't mention anything in the diet about green foods: algae, nori, spirulina flake, etc. I'm hoping it eats mainly along these lines - or it may be malnourished. The bristletooth tangs are grazers, and phytoplex isn't meant for fishes - it's more coral/clam food.
To stay healthy these fish need greens, greens, greens.
(you can check the Tang FAQ's at wetwebmedia.com for more info)