Any amount up to 2500ppm is beneficial. Over 2500 is a waste. The CO2 level when cannabis developed on earth 250000 years ago was 2500ppm. CO2 is not used at night, but can be used at any time during the plants life to increase growth rate. CO2 levels work with a balance in regards to lumens the plant can take and tds of ions your plant can process. The plant absorbs the CO2 thru the stomata which are mouthlike opening on the under side of the leaf. They not only intake CO2 but disperse water as the final stage of transpiration. I wonder if anyone has developed a method to test the ion content of the dispelled water in relation to the nute mix. Probably not but I will see what I can do. Any way I will stop rambling now

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c