http://www.youtube.com/watch?v=yElbNxnXpxo&list=FLhzFqv1U56IyLYprvyXcB6Q&index=10&feature=plpp_video
^ good video to watch. Way better to underwater than overwater. The guy who posted this video has other videos. Very descriptive.
If you have the room dedicated to flowering and you don't want to deal with co2 a passive intake would be perfect and for the size I think an 8in circular hole would be perfect. Than all you would need to do is fasten a Heppa Cone filter and you are set.