be careful what sort of epoxy you are using. I use loctite superglue gel,sets up very quickly try not to get it on the corals themselves just the rocks. The crabs arent to blame they are simply doing their job. In the wild the corals would be firmly attached to the rock and there wouldnt be any problem. To solve this issue,everytime you buy a new smaller coral/frag use the superglue gel to glue it onto a larger piece of rock than it came on,rock to rock,not coral to rock. I have a bunch of rubble down in my sump I use pieces of. Dont glue the rock to the rockwork in your tank they are a lot more difficult to move around when you get new additions. I drill a small hole in the bottom of the rock Im getting ready to glue my frag to. Then take a 2-3 inch piece of small non-flexible airline tube,superglue it into the hole,then glue the frag to the rock. Then you can stick the tube into a hole in your rockwork and be able to move it on a whim. It sounds complicated but once you have done it its very simple and you will want to do it everytime.