CO2 is commonly used for aquatic plants with fantastic results. It is also used in comercial plant production. Go to any Hydro shop for more info on this... LOL!!!
I found that my aldrovanda grew fantasicly with CO2 but only after I increased the rate of trace element feed. It is most usefull when plants are grown in close proximity with little fesh air as the plants can strip the available CO2 quite quickly. Something as simple as burning a candle can make a fair difference in a large area. I have been told number of comercial plant growers heat hot houses with gas burners letting some of the CO2 into the hothouse for the plants.
Food for thought