There is no reason to add C02 gas to your water. Remember leaves use the c02 for photosynthesis and the roots use oxygen. If you did anything I would suggest oxygenating the water for the plants and that in turn would give you a better growth rate. I'm currently using tap water for my hydro grow and I have to adjust the PH down. My water is hard water and starts with 300 ppm right out of the tap.