I would be inclined to say you were looking at a Amblygobius species as they are very commonly offered in the trade and are sand sifters. Although they and other sand sifters will often take prepared foods, I would be hesitant to put one in a 20 gal regardless, simply because they will devoid the sandbed of
any life forms within hours or days. This will also nullify any beneficial effects of the sandbed, if you were counting on any. They also have the capacity to grow quite large. Frankly, clean/white sand isn't really necessary and in a 20 gal I wouldn't worry about having something clean it. And the footprint is just too small for a cucumber or conch. A few small sandbed-dwelling snails (Nassarius) might be worth considering.
May I recommend any combination of:
-A bicolor blenny (Ecsenius bicolor) or related species, which are active and personable fishes that would be well-suited to a small tank.
-A larger tank.
-Vigilant attention to water quality, etc. to ensure you don't have problems that one might want to use a sand sifter to "solve" (often innapropriately).
See also:
http://www.wetwebmedia.com/valenciennea.htm
http://www.wetwebmedia.com/amblygobius.htm