technically what your talking about is not "fragging" if i understand you correctly because you just want to remove some dead skeletal rock from the live coral probably just to make it more astetically pleasing not to create a two seperate growing organisms right???????? If this is the case then you can safely remove as much as you want and it should be easy as the skeleton of this type of coral is very poreous and will break or cut easily. Keep the coral submerged as best you can till you have to remove it from water ( a tupperware type container) then cut it as close as you want just so you don't hit any live flesh with a side cutter pliers, dremal tool, hack saw, hammer and chisel, (don't laugh - I have done this)whatever it takes to do the job. If you have to take it out on a surface to do this I use a towel dampened with salt water. You will find that the skeleton of this type of coral cuts/breaks very easily, and all will be well. Good luck! no worries here.