Tangs are not herbevoirs (sp?) as we think of it. They graze on one form of algae or another in the wild. They don't however eat plantsas they can't digest the plant's cell structure. The thing to realize is that they are specialists. Some eat only mat algae, some macroalgaes, others eat hair algae. That makes the type of food they are fed important. While most will eat just about anything, they need the algae to thrive. Unfortunately a lot of them will only eat the algae similar to what they eat in the wild.
There is a line of Nori sheets marketed with Julian Sprung's name on it that seems to work great for me. FWIW, I recommend them. I keep the green, red, ad purple around. I've never found a Tang that wouldn't eat one of them. IME, most will eat the purple better than the others. I have a friend who has a Shoal Tang that won't touch the purple but pigs out on another color. I can't remember which one.
I think the bottom line is that if you want to keep tangs, you should find an algae based food they will eat and make it a big part of their diet.