co2 can not be used during the dark .It does no good you have to have HID lights for it to work, no floruo's. You can exhaust all you want in the dark. The only way to cut down on smell is to be air tight and an air lock entry door system. I want to go co2 but the money is holding me back right now. I'm just finishing up a small flower room and the ehaust is not what I want but will live with it for now.