that depends on the type of clam you have or want. Croceas,Maximas are found up in the reef in the wild so i always put them up on the rocks, but i dont "mount" them, if they are happy there they will grab on with there byssal threads.Derasa,and Gigas are fine on the sand along with Squamosas.but again i wouldnt try to mount them (when you say that im thinking glue) just place a small rock or half a clam shell under them.