how can i stablize the buffer?

My tap water comes from a well and has no chlorine. It is VERY hard. GH tests at 300ppm. KH tests at 180ppm out of the tap. But the KH in the tank has dropped to a reading of 40ppm!! My tank is only 20 gallons. I know a larger tank is better but that is what I have (long story). There are no...