Co2 is taken in through the leaves, not the roots. I found a nifty little Co2 generator on eBay. It is a plastic bottle with a vacuum tee at the top. You place water in the bottle, and drop a little pill that comes with it into the water. Over the next week, the pill dissolves and releases Co2. I then mount the bottle at the top of my grow area and run vacuum tubing from the tee down to just hovering over my plants. I got a 3 month supply of the Co2 pills, and the bottle for ~$10.