Croceas like to be in the rockwork. If you want to keep it in the sand(if you have enough PAR in the sandbed for a crocea), get a flat piece of rock that it can attach to then put the rock in the sand. Deseras are sand dwellers and should be kept in the sandbed. Croceas need lots of light, Desera dont need as much. Since clams are filter feeders to, its always good to have some flow around them. Just dont blast them. What is your light setup?