If this basically corner "cube" sort of tank was mentioned previously I didn't pick up on it (sorry if I missed it), that is a different kettle of fish (so to speak) for lighting.
Your fluorescent tubes will presumably be 30 or 36 inches in length, more probably 30, and I would have 4 of them if they are T8.
With T5 HO, 2 will probably do. But you mentioned 54w, that is the common wattage for 48-inch T5's (which is one reason I have assumed a stand rectangular tank). I'm not sure but I would expect the shorter-length tubes to be lower. They are still bright though, about 1.5 times brighter than T8 in the same size and same type (manufacturer, spectrum).