[Delphinus]

I do one in / one out on my canopies. This way I know I'm getting an exchange throughout the entire canopy.

Another way to go is both in or both out; this doubles the cfm basically, but I'm not sure if the flow direction is all that controllable. Probably is, but you might need to cleverly design your canopies to accomodate that.

On one tank I am using Icecap fans (DC units). They have temp sensors that contol the speed to one of the presets (low, medium, fast). Keen little units, but pricey. I think I paid $75 EACH. (They may be more now.) Check out saltwaterconnection maybe, that's where I got these.

On my other tank I am using Orian fans (AC). Bought them because they're reputed to be the quietest for their cfm ratings. I got these from Allied Electric out of the U.S. Price probably came to about $35 ea. once landed.

On yet another tank, and over my sumps and stuff when I need extra fans, I have a mixture of DC and AC fans that I just picked up from Princess Auto. They seem to have a number of different stuff and prices ranging from like $5 to $30.