Rather counter intuitively, I'd strongly go against a sand sifting starfish, for 2 reasons
1) I think the general consensus is you need a large tank that has a lot of life in the sand bed.
2) ( the counter intuitive bit), I think my sandbed became cleaner when I got rid of my sandsifting starfish. I genuinely think it was eating all the microcrustations and other life that you don't see, that were tirelessly working in the sandbed. Once the star was gone, it took a couple of months, but I saw a real improvement in the sand bed.