Ppl. use superglue gel, has to be the gel type. You have to do it out of the water. Dry the rock off with paper towel and dry the base of coral and put glue on the rock and then hold the coral down in to the glue for about 30 seconds or untill it hardins. Dont get fingers stuck. IMO best to put small rocks in a basket with corals loose on top and let them attach themselvs. Place basket in a low flow area of tank where they still get light. Some ppl. gently try a rubberband around the coral and rock as well. Good luck.