Roger, You may not be asking the right person about this...I'm kind of new to all this AP stuff.

I'm using rain water in my AP with no hard water build up. I don't think that you will be able to wash off your media. I've been told to add egg shells to buffer the PH level up...PH will dissolve the shell which are calcium. Isn't hard water residue just a build up of calcium? If so can we not just stir them down into the GB an let the PH in the water dissolve it.