Plants use CO2 for photosynthesis. They don't use carbon. Carbon is very different from carbon dioxide. I am not sure what you mean by CO2 in filters? If you are referring to the carbon, it's to remove chemicals from the water, like after using meds or something. It's only good for a month anyways, so you should change it or take it out after that period of time.. or it may release "stuff" back into the water.