At night plants actually release some of the Co2 absorbed during the day. This is why I run a closed system with my flower room exhausting into my veg room. While flower room is dark, veg room is still illuminated for 6 hours and receives released Co2 without running my tank system. Though the veg room is smaller, vegging plants can endure higher levels of humidity, and the veg room has its own exhaust set at a higher humidty level. The veg room exhaust into a room with a dehumidifier (portable a/c in Summer) from which the flower room receives intake. I swear by the closed system for its efficiency and enhanced stealth.

Peace