Also, I believe potassium carbonate (K2CO3) adds ~33mg/l K+ along with each 1 degree of hardness added by the CO3 just like K2SO4 does. It works well but you have to understand and want the K+ ratio. lots of extra K seems fine, but plants don't need KH so it's just a pH buffer issue. If you have two or three degrees in your tap I would not bother raising it. Over four degrees can limit some plants. Otherwise K2SO3 is supposed to be a good way to raise KH.