The Current USA T5 bulbs will work in any T5 fixture, as long as you are using bulbs of the same wattage as those currently in your fixture. T5 bulbs come in two categories, High Output (HO) bulbs for Nova Extremes and Normal Output (NO) for Nova Retrofits.

T5s are similar to the Standard, HO and VHO fluorescent bulbs commonly used in aquariums and manufactured by URI and Coralife, they are just thinner. T5s are 5/8" or .625" in diameter.