The Tigertail Cucumber - Holothuria hilla - is an excellent sand sifter for tanks using a DSB (Deep Sand Bed). However, it will burrow under your live rock as it processes sand looking for food, so you may have to readjust your rockwork from time to time, or support the foundation with PVC to avoid its collapse. The main risk is when a cucumber gets too close to the intake of a powerhead or filter. Their death can wipe out a tank, so always know its location. Tigertail cucumbers will self-reproduce by tearing themselves virtually in half. My own personal experience resulted in a minimum of seven cucumbers from the original single purchase.
Look closely at the first picture and you'll see the cucumber has a series of appendages that it uses to mop the sand in search of edible food. Clean sand will exit the other end of the cucumber in small pellet-shaped clusters, which break up easily in flow. The second picture was added for fun, because the creature picked up a small coral branch as if to say "look at me, I'm a snake!"