Well.. its not really for making caves.
I just want to glue together a rectangular "flowerpot" to put some substrate and a plant in. Its not going to bear any weight or be a hazard (tumbling over and busting my glass).
I had some silicon lying around, but it specifically stated "NOT FOR AQUARIUM USE"... so.. thats out of the question.
I'll go to Wal mart and find some cheap silicon or epoxy