What I do it place a blob of superglue on the frag, by taking the frag out of the water and drying the spot where I am planning on gluing. Then when the glue hits the water, it will create a skin.... When you get the frag to the spot where you want it, wiggle it back and forth, breaking the skin.
For SPS, I usually superglue the frag to a rock, then use epoxy to mount the rock.