Hardness is actually dissolved minerals in the water. You can't add something to remove them, they have to be filtered out, that is basically what a ro unit does. Using straight ro is a bad idea, as hardness stabilizes pH. You will start out with a pH of 7.0, as a little waste accumilates the pH usually dives, sometimes severely. Many aquarists will mix 75% ro with 25% tap to get softer water with lower pH. This leaves enough hardness to stabilize the pH.