Some people seem to swear by the little PC case fans wired to a AC adapter. How those are used in conjunction with a carbon filter is all up to one's ingenuity and imagination, really.
Myself, in what limited growing I've done, I've just used a activated carbon 'closet odor' controller. You can find them in Walmart near Housewares, where the hangars and such are. Just a slim plastic container with a black carbon gel in it that dries out over time, absorbing odor as it goes. I only had one plant going at the time I used it, though. Mileage may vary, and such. Worth checking out, if your grow isn't very big.
No idea if doubling or tripling up on them makes any difference. Just my two cents, ya know? Heh.
