Baking Soda - 1 tsp per 5 gallons will raise KH and PH. Make sure you dissolve it fully and go slowly over several days.
As Fort said, there are additives that you can buy that will maintain your levels at a certain range.
You only need to change your hardness if the fish you are buying was used to that range. Don't change it unless its lower than 3KH and your dosing massive co2 to drop the PH.