Growing plants will use whatever source of carbon they can find. If CO2 ppm is too low they will start to use carbonates for carbon, and that reduces the KH. If you have pressurized CO2, as you said, you should be increasing the bubble rate so you get a lot more ppm of CO2 in the tank. Don't go by the KH/pH tables. Just raise it a bit every day, while watching the fish carefully looking for clustering at the water surface, colors fading, etc. which show they are uncomfortable with the high CO2. When that happens, back off on CO2 until they are again comfortable. That is the CO2 rate you need in the tank. Check pH, KH, bubble rate, and use those numbers as your targets to keep a steady CO2 ppm.