does your hex have a top, or is it open?
you could make a pendant out of one of those clamp-on shop lights (the kind with the round "funnel shaped" aluminum reflector), just spray paint the outside of the reflector high gloss white or black, and hang the light above the tank using it's cord
for a bulb, use the most powerful compact fluorescent floodlight (of the daylight variety) you can find
if the floodlights aren't available in daylight, just get the most powerful daylight cfl (compact fluorescent light) you can find, and let the reflector bounce it down into the tank
if your tank is covered, I'd need to know more about what type of bulb the existing cover takes, you might be able to just screw a cfl right into it
of course, if you have a bigger budget, you can visit www.ahsupply.com
and get their one of their 13 watt kits, those bulbs are so small you can put them anywhere