The bigger the better. Anthia's will school in a 72 but that's definitely on the small side and would be better suited to a 125 or larger for a proper school. If you wanted the Pilot fish which are really active and fun to watch, you'd need something in the tune of 180 or larger. The bigger the better as far as tanks go, because what happens is with a lot of space, it brings out their Schooling instincts and have to school for better protection from predators (even if there isn't any true predators in the tank). Also for schooling fish, I notice if you have a fish or two that is larger and active such as a larger tang or large angel, it'll help maintain the schooling instincts a little better whereas if they don't have anything like that, they'll begin to lose their schooling tendencies and sometimes will even develop territories, I think this happens more with Chromis than other schooling fishes.