You only need to add CO2 when the CO2 level is low, if you have high light levels. You should test your CO2 level to find out what CO2 level you room is at. You should also figure out your watts per square foot that your room has. Your plants only benefit from using (adding, injecting, ect.) CO2 if your watts per square foot are above 40 watts per square foot. These are the optimum CO2 levels for different plant stages: In the grow stage plants do not need a high level of CO2, what is in the air is usually okay (around 400-500 PPM normally). In the bloom stage you need a higher level of CO2, around 800-1000 PPM. Some plants like peppers & tomatoes like a higher PPM. Anything above 1500 PPM can harm your plants, taking needed oxygen away from the root system.