Unfortunately, I think the Naso and N. vlamingi will quickly outgrow a 180. My favorite tang, and one that stays very reasonable sized, is the chocolate or mimic tang, Acanthurus pyroferus. I'm thinking of the lemonpeel angel mimic. It starts out yellow, but the adult coloration, while subtle, can be really spectacular. The finnage is the prettiest I've seen on any tang. This species tends to be a good community tank fish, not too aggressive, but assertive enough to take care of itself with aggressive tankmates. I had one for three years with an emperor angelfish, a harlequin tuskfish and a zebra moray (as well as some smaller fish). Never a problem...