You can use epoxy putty. But it's kind of a pain. Another thing to try, is put a big gob of super glue gel on the bottom of the rubble or frag, allow it to air-dry for a minute, so it skins-over. Then gently lower it into the water, and mush the glue-bubble into the rockwork where you want to stick the frag. This works pretty well with the really thick gel.
Nate