i had a sand sifting star and he lived for a very long time in my 29 gallon, but one day i noticed his legs were starting to vanish then i found that at night my urchin was munching on his arms. a few weeks ago while i was looking at my tank i noticed that he hadnt moved and sure enough he was dead.
i have a 29 gallon too and i have 5 big snails and many little snails and i have an emerald crab, 2 scarlets, 2 peppermint shrimps, & 2 sexy shrimps, but i will be adding a few more peppermints soon. hth