Assuming you do large water changes, mixing tap with RO/DI is the easiest way to gradually decrease and maintain a target KH/GH IMO. Keep in mind there are very few fish and plants that need such treatment, of course. Just as a stable pH is better than an ideal pH, stable KH (and maybe GH) is essential.
If you want to lower GH alone for some reason, you can use "sodium-free water softener" (KCl), if you want. Also a good potassium fertilizer. I bought 40lbs for ~$15 at Home Depot. Good for laundry, too.