Crazy glue is used by surgeons to close tissue together, so it is safe to use in your tank. I use superglue type gel and lately I've become a big fan of CorAffix by Two Little Fishies. The long nozzle doesn't dry out, and makes it very easy to get it where you want it. It has a rather slow curing time (several seconds
) so I can glue out of water and re-attach a frag to a plug that it was knocked off.
For gluing rocks together though I would use a 2-part epoxy like Aquastick or the very similar epoxy I saw in the plumbing section of Lowes for half the money.