My personal opinion is to size the tank to the natural behavioural habit of the fish. What I mean by this is, would you put a fish that normally swims over a vast area of reef in a school (e.g. yellow tang) in a 2ft cube by itself? 8O
Better suited would be cardinalfish, gobies, blennies, damsels, pseudochromids, etc that live in small, defined territories.
Just my personal preference ...
Aloha!
JCD