i have pretty hard tap water. its at ph 8 so i use the co2 to drop it to 6 - 6.5. ive noticed too that with the co2 my crypts can take high light and not melt. all plants require co2. low co2 levels stop plants from being able to absorb their maximum potential energy form the nutrients and light in their environment.It seems like you really do not have a whole ton of plants that require co2. Is there some plants I am missing in the pictures that need the co2?