super glue/krazy glue, gel. it has to be the gel. but you take your rock out, dry it, then take your coral out, dry it, find a place where they fit together well and then attach them with a dab and proceed to hold them out of the water for a minute while the glue semi-dries, then put the coral back under water and hold it there for another minute. if you dont do the last step, the glue will stretch and look gnasty.