Clams and scallops are definitely out. Most tridacna clams require metal halide lighting as they are totally photosynthetic. The only giant clams that don't require high lighting are squamosas and they get to be up to 20" long which is totally unsuitable in a tank of your size. Scallops are total filter feeders who are sadly enough doomed to die in captivity and should be left in the ocean..........stick with mushrooms, zoas and some of the leathers such as toadstools, fingers, and such like kenya trees, also xenias and other polyps....