I think i have used this gel without any problems. You don't have to take out rocks.
-> First use the turkey baster to remove all the sand/other junk where you want to glue the frag.
-> Take the frag out of water and apply gel to the frag and then hold it under water where you want to mount it.
-> Turn off all the pumps and other circulation and hold the frag for 30 seconds and it should do it.