I suppose there is cement that you could use... however it would take weeks if not longer to cure outside of the tank before it is usable... and it's probably just not so great policy... I do wonder though about using the foam that people use for backgrounds to work as epoxy and get stuck in the pores on the sides of the rocks that are touching. I dunno.
I wouldn't really suggest anything other than epoxy, zipties, or plastic/acrylic rods.