Tek t5 in a 150

what do you want to keep? for a 72" long tank I would go with 3 MH's or a 72" T5 fixture otherwise you will have dark ends.
 
You will need a 72" Fixture, It depends on what you want to keep, Metal Halides probably for this tank would make more sense.
 
I would agree, but wait to see if anyone with a 72-inch tank that has actually used T5s reports whether it is a satisfactory light source for that tank so that you could keep even light-demanding corals successfully. MH would be for sure, but there's probably still some question about T5s being the right thing and you may be limiting yourself significanlty by going ahead with those.
 
MH is most likely the way you will have to go. If you want to go t5, maybe you can hang two 36" fixtures above the tank, that would give you 72", or do a retrofit with 2 36" bulbs going across.
 
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