during the night, plants dont carry out photosynethesis which is a process in which they requie CO2. they carry out respiration during this time which requires oxygen. so have your co2 burner running when the lights are on i guess. thats an educated guess, i'm not a professional grower, i'm a newbie so wait for more info that ppl are sure of. i cant see why it would be to be on whilst the plant is in the dark though. gl