As a Zebra tang I would say the minimum size to display most of its natural behavior is 180 gallons, and loosely decorated.
Honestly, after keeping different types of tangs I have to say that I'm starting to think they really belong to public aquariums or the ocean. The public library near where I live has a roughly 3,500 gallon public aquarium with mostly tangs, including both large and small species. all they do is swim around and around in relaxed manners all day long, and that's the natural behavior. they just don't display that kind of behavior in a typical home aquarium, unless it's at least 10-foot long.