Thanks for the replys everyone.
I do actually have two fans - one is a 6" 449cfm Vortex and the other is a Dayton 4" 265cfm squirrel cage. I was planning on using the Dayton for fresh air intake but if passive would be sufficient, I can use it for cooling the light and put the Vortex on the Carbon Filter for scrubbing.

I'm gonna go check out Dusto2k3's thread again...he had a badass hydrohut but I think he used three fans in it.