Is there a set of rules as to what is placed on the sand/bottom and what is placed on rocks? I know with clams some, like croceas and maximas, are placed on the rocks so they can get enough light and the others derasas, squamosas, gigas' are placed on the sand because they do not need as much light and because they can get too big to be on the rocks. I was asking Mel about plate corals and she said they should be on the sand because they have a fleshy underside that needs the softness of the sand. There may be some that "œyou want to place on sand" because the sand can act that a boundary and minimize spreading (they would tend to spread on rocks). Are there any other rules?