Depends on the size of the grow tent I guess :] I've only got a small one so I won't have to use a whole lot of CO2 to raise the levels up.
Seeing as the temperature can go up to 95 degrees Fahrenheit comfortably with CO2, I won't need to have my extraction fan running at all or if I do, it'll be on a lower setting, so that's less CO2 to worry about.