The minimum co2 levels necessary for a successful harvest is 350 PPM. Fresh outside air is usually around 400 PPM. Anything lower than 350 PPM will reduce the yields and quality of your end product. Co2 can be injected as high as 1400 PPM with the lights on.
Co2 levels can rise at night due to plant respiration. It’s important to always make sure that your lights off co2 levels stay under 500 PPM during the duration of the growing cycle. If necessary, use a fan and carbon filter to exhaust the excess co2 from the growing area when the lights are off.