CO2 is not called for in veg. It must be a closed room as TurboALLWD said or your just throwing money away.

CO2 is really for flowering, should only be run when the lights are on. A common setup is to use a controller, set ppm to 1500, and top temp to be 90. Then, the controller will come on and dump the air in the room, and when finished, bring ppm back to 1500. Plants like it hotter when there is CO2, and can tolerate and thrive in higher temps.

CO2 should be NOT be run the last 2-3 weeks as your buds will not swell.

hth