I remember that when I tried to put the clam at where I wanted in my tank (see my tank picture and you will understand, I have a little "platform" designed for it), it kept falling to the sand. But I kept putting it to the place I wanted. After about 5 to 6 tries, it finally stayed there and attached to the rock. Also, I think that some clams can be put on rock while others prefer the sand. What kind do you have?