If you start pumping CO2 into your tank, eventually you're going to hit a state which is called pearling. In which CO2 bubbles will be emitted from your plants. Once you have so much CO2 in the water that the plants themselves start giving off Oxygen. This is of course, with enough light and CO2.
So, if you start pumping CO2 you could actually kill the plants. They need Oxygen too. So I say play with the settings. Increase/decrease light/CO2 until you can get the pearling effect.