I run the C02 during the lights on time and turn the exhaust fan back on and run it until the lights come back on. If you dont think your exhaust fan is doing anything turn it off while the lights are on and watch the temp gage climb. 

From what I have read if you keep your temps under 80°F C02 doesnt really do much good and its not easy to keep your temps high if you can lower them. Seems to go against the nature of things