If you feel you must have sand sifters get Nassarius Vibex snails. They are small and will sift around under the sand coming out mostly when food enters the tank. It's quite fun to feed and watch them emerge from the sandbed.
Otherwise I agree with Mike your tank is too small and to young for a sand sifting star or a Sleepergoby. Both would starve slowly in your tank.
Your best bet is to learn to live with a less than perfectly white sandbed like I eventually did. It's not a bad thing, even though we tend to see it as unsightly.