It's true that tangs can eat meat, but generally their diet should consist of a lot of veggies. I'm just going off of Live Aquaria recommendations, I'm sure there are many other opinions:
"Although Tangs will eat meaty foods along with the other fish in the aquarium, it is important that they are offered plenty of marine based seaweed and algae. This will strengthen the Blue Tang's immune system, reduce aggression and improve their overall health. Offer dried seaweed tied to a rock or use a veggie clip, and feed at least three times per week. Sea Veggies, Seaweed Salad and Ocean Nutrition are all ideal products and are very easy to use."
Goober35, I'd feed macroalgae/seaweed/lettuce at least every other day. Of course you can feed every day as well, as in the wild, tangs eat throughout the day.