KH is the measure of Calcium and Magnesium and other hard ions associated with carbonate (CO₃^2-) and bicarbonate (HCO₃^-) ions contained in a solution, usually water. It is usually expressed either as parts per million (ppm or mg/L), or in degrees (KH - from the German "Karbonathärte"). One German degree of carbonate hardness is equivalent to about 17.8575 mg/L.

Ways to increase KH:

• Adding sodium bicarbonate (baking soda). One teaspoon of baking soda added to 50 liters of water can raise the KH of the water by approx 4 ^OdH without a major affect on pH.
• Adding an air stone to increase surface turbulence driving off carbon dioxide (CO₂).
• Adding commercially available products to increase buffering capacity

Ways to lower KH

• Injecting carbon dioxide (CO₂).
• Use reverse osmosis (RO) water. You can mix tap water with reverse osmosis water to achieve the desired KH.
• Adding commercially available products to decrease the buffering capacity.