Understand there are T5 and T5HO. and they are different beasts.
T5 use 7 watts per linear foot and T5HO use 12-16. They are both 5/8" diameter and use different pin spacing from T8 or T12 and will not fit in those fixtures. T5 offer efficiency gains over T8-T12 and the ability to use a very effcient reflector. They are usually available at Lowes or HD. T5HO are like night and day compared to T5,8,12. Whenat he'd to a good reflector they are on par with MH. They are not available at those stores.
T5HO are available in many spectrums and a fixture with 4 or more bulbs can be fine tuned for the specific phase of the life cycle. They are available in 2', 3', 4', and 5' lengths @ 24, 39, 54, and 80 watts respectivley. If rigging up bulbs yourself, reflectors can be purchased online or locally at saltwater fish stores.