You are supposed to run Co2 @ night, that is when the plants use it!!!
sorry but i think you have it backwards; plants don't use co2 at night as there is no photosynthesis occurring; co2 is the limiting factor in photosynthesis hence the upping of co2 in grow rooms to overcome this limitation... at night plants use oxygen to make atp not co2
co2 controllers have photocell switches for this very reason