after you bleach, rinse really well. don't want to mix a base with an acid...
once rinsed, cover the rock in ro/di and add enough muriatic acid to make the water boil like a witches brew. it boils because the acid lowers the ph of the water enough to dissolve the surface of the rock, thus releaseing bound po4. it will boil for about an hour. once it's done, add enough more to do it again. then rinse and you're done.