Wow some of the post amaze me. Not to discredit anyone, every situation is different but from my experience CO2 is the greatest thing since sliced bread. I grow indoors and for 3 years didnt use CO2 and this last harvest we put in an environmental control system linked to a 50lb CO2 tank and regulator. What this does is keep the room at a constant 1500-2000 ppm of CO2 and YES MY PLANTS GREW FASTER AND DOUBLED THERE SIZE compared to previous grows.(2ft plants in about 3 weeks) If your enriching your room manually it will not work the same. YOU NEED CONSTANT CO2 LEVELS OF 1500ppm to see sufficient results. And this is applying to INDOOR grows not OUTDOOR. For the record my room is 8x8 so no it doesnt have to be a HUGE room to make it worth your while trust me on this one. Google this stuff and do some homework, you may be amazed at what youll find