Not a huge fan of epoxy for holding rocks together, it just didn't seem to work all that well for me. If you do it though the trick seems to be pushing the epoxy deep into the cracks of the rocks to lock them together. Some epoxies have other ingredients like "Steel" or other metals, just make sure you get one without. I use epoxy for large coral colonies and place gravel rocks at the base to hide the epoxy, but everything else I use it superglue. The BRS superglue is great out of the water for frags, but under water isn't any good. The Boston Aqua Farm superglue has worked the best for me underwater and for the size tube you are getting it is cheaper then homedepot.