i personally don't really think CO2 is needed unless you have a room full of weed that is depleteing the levels. i don't think a single plant under CFL's really needs CO2. i tried running CO2 all winter and i didn't see any major difference. my levels were at max for 12 hours a day. i think a lot of people are getting sidetracked on CO2 when they should be focused on other things. CO2 is more a "final tweak". adding small doses here and there really isn't going to do much. this is just my opinion.