How long has your tank been established? Sand sifting starfish are great BUT they need an established sandbed to live. if they don't find food they go unde the sand and die and then decompose and pretty much can nuke your tank.
Maybe try a sand sifting goby? they are known to eat other foods not just the stuff in the sand so it might be a safer bet.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.