Really simple. Get a piece of 'conditioned' limestone from your lfs, and if you have no room for it in your tank as it stands, just use it as the foundation of a rubble heap in your sump---nothing dense that will trap crud, but an anchor place for bits of life that may be useful in some redesign of your tank. Put any dead coral skeleton, old shells, etc, into this pile, and you've always got pieces you can use to shim a larger rock, or if tiny, just a lump you can glue a frag to, to enable you to secure it in your dt reef. The older it is, the better it gets, and it also provides a place where pods and other life can breed and multiply. Just keep it open as lacework to assure you don't get crud accumulating. Water should move in it.