the T5 and T8 luminaires are different. the t5's use 5/8" bulbs, the t8 use 8/8"(1 inch). they are replaceable with the same size bulbs as many sites sell the bulbs, but i doubt the t8 can be replaced with t5 or visa versa as they are probably different wattages. they are meant to be very bright. the t5 luminaires are more expensive as they have more bulbs i.e four in the large luminaires. the do marine and freshwater versions both use different types and wattage of bulbs. you'd want the marine. i've been mulling over which one to get and have been told that the T5 are really bright, the 5ft one has 4x54w bulbs, probably too bright for my needs, thwey are probably really good for deep tanks. all of what i'm telling you is what i've found out along the way, but i've never tried them so am only speaking in theory.
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