well when they tell you that you need a 3/4" bh, its actually the measurement of the inside of the bh. so i a 3/4" bh will need a 1 1/2" hole for it to fit into with the threads and all.
man i love your 90 cube. it really makes me want to upgrade from my 30sumthing cube. oh yeah, hows the hair algae problem coming along. i hope you kill it all cause your tank is awesome.
-Stan