Gel Super Glue. It works great and you can buy a pack of 5 for about $6. There are also glues that are made for it. The putty stuff doesn't work very well. I have been using super glue for 2 years. It has to be gel. Put the gel on the on the coral where you want it to stick and put it on the rock. Just hold it in place for 30 sec to a minute and you are good to go. If it doesn't stick add some more gel and place it again. You can even use the tube under water but it is easier out of the water. Hope this helps.