If the canopy is sealed fans blowing in with exit holes up high in the back center or better yet in the lid or roof of the canopy work best. You take advantage of convection that way, heat rises.
The suggested one in one out only gives you the CFM value of one fan, the second fan only moves what the first one fed it, it adds nothing to the flow. Fans blowing out also tend to gunk up quickly from the warm moist air. Fans blowing in move cooler dryer air so last much longer and stay quieter over time.
I use two 120mm (4") Vantec Stealth DC fans in a completely sealed canopy, one in each end blowing in. The roof of the canopy has equal sized or larger holes directly above the reflectors so heat escapes but light does not. The fans are controlled by a variable voltage DC power supply (wall wart to some) that can be purchased at Harbor Freight, Wal Mart or just about any electronics shop for less than $10, I paid $5 at Harbor Freight and its been in operation 6 years now. The fan speed can be adjusted by raising or lowering the voltahe with the turn of a small knob or sliding a switch. At 12v the fans move 53 CFM each which is 106 CFM and my 100G canopy at 16" high contains just under 10 cubic feet of air so I get 10 air changes a minute with almost zero noise. In the winter I slow them down to 10.5 or even 9v and its dead silent. The lighting is 2x250w SE MH and 2x140w VHO super actinics so I have plenty of heat to remove and this setup works great. My chiller has not run since April 2008 and the tank stays at 79-81 year round with only these two fans and a simple WalMart clip on fan over the sump in the summer months. The clip on is on a Ranco controller and comes on when it gets to 81 and goes off at 79 and maybe runs 8 hrs a day. The canopy fans are on the same timer as the lights so run only when the lights are on.
