We've just started using Seaweed Selects for our tangs, and they (after 3 days of ignoring it) are starting to chow it down. We are letting it soak in Zoe (a vitamin suppliment) overnight in the fridge so that they can be super healthy.
We have also put the little tips off of brocoli into the water, and they DO eat it. I am not sure how good or bad it is, and we've only done this once. I have heard of some people making a 1 cup (a week) batch of a veggie mix that they put into a blender or food processor to cut up a little bit (not turn into a pulp), and they feed that.
Our fish like like plants the best, and love nibbling on them all day long. I have had to put a small refugium in just to grow more plants (calerpa).