2Hr Aquarist Commentary: For increasing KH levels without increasing GH, use Potassium Bicarbonate or Nilocg's KH booster. Increasing carbonate hardness/alkalinity increases the pH of the water. This is done more for livestock than plants as most plants grow better in soft water rather than hard water. (Exceptions being Pogostemon helferi, Valisneria species).
Note that this is a 3rd party product sold on Amazon that I have used or have experienced to a degree that I'm fully comfortable to recommend. If you do buy the product through the Amazon link below, I do earn a small commission (at no extra cost to you), which helps in the running of this website.