most people i know use a layer system to glue them down. They put gell glue, then a ball of two part epoxy putty, then more glue. I have had some luck with attaching the frag to a small rock out side the tank in that way, and then glueing the rock to the main rock in my tank. It seems to work better than glueing the frag under water, because there is a larger glueing surface.