I've considered CO2, but it seems to me that it's just as cheap to just add some more square feet of plants and grow more, if quantity is that big of an issue. I also don't want the hassle of a separate ventilation route just for my lights and another one just for the plants/CO2. Not saying that it's not worth doing...for a few people. But for most folks, it doesn't really seem very worthwhile to me. Just my two cents, and with that and five bucks maybe you could buy a coffee.