I have used hot glue before and it does work. It is completely safe (chemically) once it dries. You have to add a lot because once it gets wet, it softens and doesnt hold as well. Lastly, make sure when it dries there are no strings of it hanging off cuz the fish try to eat it. But like others...
What difference does the boiling make for the coconut? I cut one in have, then cut an opening in the front. I placed it down rightside up, not sideways. I also drilled 3 holes in the top so air bubbles can escape.
P.S. It looks great with a bubble tower and blue light inside! :D And my...