I just bust off pieces of the rock they are growing on. I never glue corals directly onto stationary rocks in mt y tank. I put them on small rocks and glue the small rocks to the larger ones. When its time to frag, i give the smaller rock a good tug and it comes off pretty easily. I then break off small pieces. When the rocks get to small to do that anymore i just glue that to a bigger piece again and start all over. Heres a pic of mine.