Personally I'd get a 24" fixture and a 24" tube 5.0 but I like full coverage of uvb on my cages. DIY cages has some real nice 24" dual fluorescent fixtures in 24" lengths. That way you can add a plant bulb to the mix and keep your plants alive and looking good. The fixtures are pretty reasonable considering they come with seperate toggle switchs for each bulb and they come with a reflector already built in. You can get cheaper fixtures at hd or lowes but they arent going to have seperate switches or reflectors from what Ive seen at my local stores.