Bullets, Diamond, and Gold Head Sleeper gobies are the best breeds of sand sifters I've ever seen in action.
All of the ones I've had were nonstop workers, constantly gulping at the sandbed.
It's the first thing someone new notices about my tanks.
"Hey! Did you see what that fish is doing?!?" lol
Like Fluff mentioned, they aren't really the right breeds for smaller tanks.
All 3 breeds get quite large, 6"-7", so a 30-55gal tank would be a minimum.
If you get one from the LFS
, I'd make sure to pick one that's actively sifting the sand to help make sure that it's happy and healthy.