At AquaCave, we offer some of the best selection & best prices on T5 Fluorescent Aquarium Retro Kits for your marine, reef, and freshwater aquarium. T5 fluorescent bulbs are 5/8" diameter fluorescent tubes rather than the standard 1-1/2" diameter. This slim profile makes the T5 Fluorescent system more efficient than fixtures that use standard fluorescent tubes. T5 fluorescent bulbs also put out more light than standard fluorescent lamps, power compacts and VHOs.