Superglue gel works well. A little trick to help the frag and glue adhere underwater to the rockscape is to first rub some glue onto the rock, place a dab of superglue gel on the end of an old peg. Then just rub it somewhat vigorously on the spot you want it on. It seems to leave a thin layer well adhered to the rock scape. Then add a drop of superglue gel to the bottom of the frag and place it. The bond seems easier to make this way.
I always suggest all frags should be removed from their original plug, inspected for pests, dipped in Bayer or your preferred insecticide and preferably quarantined if you have the capability or if not then placed in the display. There is a propensity for nasty pests at the base so that is why you remove a frag from its original plug