I'd recommend a little bit of epoxy putty, to get it to fit where you want it to. I've had the best luck with a bit of super glue gel applied to a clean base, a small lump of epoxy putty, and then a bit more super glue gel to adhere to the aquarium rock. Helps even more if you gently brush off the mounting location and frag mount with with an old toothbrush first to remove any surface film/algae before applying the super glue. The old frag mount will be overgrown in no time.