Thru all of my travels on educating myself on growing I found this tidbit I need to add to this thread....my grow room is a spare room/lg closet and a night when lights go out I close the door....from what I read , if you wait to open the door 30 mins after lights come on.....the plant makes use of the CO2 gathered in the room from the night before...easy and free way of increasing levels...of CO2!!