fans needed for 2 or 4 T5 48 in. in a DIY hood?

I think some slits cutin the top would be sufficient. T5's run pretty cool, and it's a 75 gallon anyway, I doubt it would raise your temperatures much anyway.

Agreed, no fans would be needed.

Cool enough to touch, as cool as a T8 48" 32 Watt is to the touch or much warmer? I'm thinking 54 Watts would produce a lot more heat than 32 Watts.
Thanks. Not so worried about temp of water rising, but about minimizing heat in a room that also has computers and monitors to keep energy needs down.

Another thing that will help keeping underhood temps in check is to mount the ballast(s) outside the canopy.