For that fixture, I recommend Sylvania, 23 watt, daylight, "now smaller than ever", spiral fluorescent bulbs. They fit well and won't stick down into a really full tank.
When I used the Sylvania's I think mine rubbed right up against the guards so I still took them off. It was from a tank that was probably purchased in the early 90s though so they may have been redesigned.
I have another tank hood that has different guards that are getting on my nerves. You have to keep the gauards on or the bulb part will fall into the water and all i can fit in there are these small skinny ones that arent doing very good.