When I first got my tangs, I kept feeding Julian Sprung's Sea Veggies. The tangs devoured it. However, they also made a mess of it. They would tear it off and it would float away, only to be caught in the sump.
I will only feed that rarely now. I feed Spirulina flakes now, and the tangs, along with everything else in the tank loves them. And not one flake hits the bottom. The tangs are brightly colored, happy and healthy.