Co2 won't do a thing to lower GH or KH. It only lowers your ph. It'll lower your ph just about as much as you want it to. But there is a limit to how much your fish can take before they get uncomfortable or die. Levels up to 30-35 ppm of co2 should be safe for most fish. Most non co2 supplemented tanks have a co2 level of 1-2ppm, which is equivalent to the level of co2 in the atmosphere.