I've seen tank eat lettuce, though they seem to like seaweed products better. Try that one, if not, I agree they're simply carnivores.
Buy one of those veggie clip, get the nice one with the little waffle-like clip, so the tangs can eat it through the little holes without having to bite it off the clip. Most of the time, the seaweed got torn apart and sucked into the filter, resulting no food for the tangs
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sebae_the_clown