Some people like to say to give the Tangs Romane lettuce, brocolli, etc. I don't really agree with this, cause they don't eat much and it just makes a mess in the tank. Plus, these foods don't really have a whole lot of nutrients that the fish can use...
Try giving them a great seaweed. Seaweed selects has some great stuff that fish love to devour...
Also, I have had some god awful experiences with lettuce clips
You put a small piece of seaweed or something in there, and WHAT A MESS!!!!!!
Now, twice a day I just break off 2 very small pieces of seaweed, let them sit in some tank water for a few minutes, and throw them in.
The fish devour it in no time, and there is just about zero mess
Also, Spirulina flakes are great. They love 'em. Your Tangs will also love some meaty foods like Brine Shrimp, which is fine as long as they're getting their vegetation as well...
Just my opinion...
Well, a lot of people on this board know a lot more, so I'll shut up now and let them answer