Plants can only take in a small amount of c02, and the amount of co2 that is in the air already(which is 300ppm) is already way more then what a plant can take in. The only thing that you can do to keep co2 avialable to it is to introduce fresh air and exaust out old air. This will keep the co2 lvl circulating in the room and at a optimal lvl 300ppm.
You have to think about it, if plants can really take in alot of co2 then why do we have a blanket of co2 in our atmosphere causing Global Warming. Inject more Co2 is a waste of money and its only there because huge companies can make it into a cash cow.