I'd go for the much smaller "Turret/humpback/pyramid/hovercraft" boxfish, Tetrosomus gibbosus. They only get about 6 inches long (in an aquarum anyways), and are much less likely to nuke your tank in a fit of panic lol. Some of the cowfish species, especially the common one, can get almost two feet long in the wild. Even in a tank they get 12 inches or so...which is a hefty fish for a 65 (think football with tail and horns)...Plus they eat a ton, and produce lots of waste.