How to I raise kh in a tropical freshwater aquarium without raising the ph?

pH is related to [H+]. kH is related to carbonate ions [CO3 2-] and bicarbonate [HCO3-]. They can be correlated, but don't have to be necessarily. Both potassium and calcium carbonates (KCO3 and CaCO3) do not have hydrogen, so they won't raise the pH like baking soda will. KCO3 is a food additive and CaCO3 is usually sold as lime, although it can come in other forms that are less soluble, like seashells. Crushed coral is a common one that's used.