You can add any starfish. serpent star, brittle star, blue linkia or even a sand-sifter. Chocolate chip stars are ok as long as you have no corals because they tend to eat them! there are many different types, but be aware that they often demand optimum water conditions. They can disintergrate and die off if they get stressed out or don't eat.
They are not very active during the day really except the blue linkia. The sand sifted is said to eat good things in your cc or sand bed; personally I think that at least one is ok in a large tank others disagree. Good Luck.