You can boost GH with: http://nilocg.com/gh-booster-1lb/
This adds both calcium and magnesium, which are both essential. And, you can boost KH with plain baking soda, Sodium bicarbonate. KH isn't critical, but you need some in order to buffer the tank water against pH fluctuation due to weak acids, such as tannic acid. A KH of 2 degrees, or about 40 ppm, is adequate. A higher KH will also raise the pH when you are using CO2.