[MrSquid]

A lot of times retro is the way to go with a canopy. The fans/vents on fixtures are often on the ends of the unit, and those ends are going to be pretty tight to your canopy. Even if they're vented off the top they could be close. Looks like you're thinking of buying used, but if you go new you can save a good sized chunk of change by going with retros.

For cooling, pick up some 4" fans and secure them to your canopy. This way too you can get the fans YOU want and make sure they're quiet - you're not stuck with whatever noise the fans in the fixture make. Coralvue makes very quiet fans with thermostats that automatically regulate the speed the fans runs at. In my canopy I have 5.5" between the tank top and my MH's and almost 6" for my T5's. That's way more room than I'd have if I'd gone with a fixture.

Same goes for moonlights - you can purchase moonlights pretty inexpensively. I got 3 from Drs Foster and Smith for about $60. They just screw into the canopy and all link together - one cable for power.

I felt the same way when I started looking at lights - wanted a fixture to make things easy and clean. But once people started explaining why retro is the way to go if you have a canopy, I changed my mind pretty fast.