psusocr, you're arguement of " my foxface uses all 6 feet" doesn't work at all, go buy a b/g chrmois, or a damsel, and they'll use all 6 feet of the tank, that doesn't mean they need that much room.
and Anthropo, you can't really trust books...Mrine Fishes by Scott Micheals says you can have a 15 inch volitiotan in a 55 gallon, they're often inaccurate.
personally, i think it would be a toss up, i've seen foxfaces in 75's, but they were only about 6 inches, so i dunno., personally, i wouldn't recomend it...