I have had this idea of a graph that shows what plants need to grow and in what amounts. I don't know if it is even close to being correct so be easy on me. lol. Let me post it here. It is kind of rough and might be too confusing.
CO2 isn't quite like the other things on the chart. High CO2 works well with light from low to high. But, with high CO2, comes faster growth, requiring more nutrients. If there are no fish/shrimp in the tank, there is no limit on how high the CO2 can be. I'm not sure how to modify the chart to show that.