Its most likely a Valenciennea or Amblygobius species goby. They are the most common sand sifters. Here is a page full of various sifters
http://www.liveaquaria.com/product/s...fm?pCatId=1851.
The Valenciennea gobies (orange spotted diamond, goldenhead sleeper and a few others) are very active sifters, they tend to say close to sandbed, and are very good jumpers.
The Amblygobius gobies (most common is the dragon/banded) is also a good sifter, will get a bit larger than then the sleeper gobies, they tend to get a mouth full of sand then go over a favorite perching spot to sift which buries whatever they like to perch over(can be a coral), they are also jumpers.
Sand sifters will keep you sand very clean, but they will move the sand where they want it so you will have to decide if you can put up with that.
The two spot goby and the rainfordi goby are smaller sifters but have poor surviveability in aquariums due to feeding issues. A healthy, feeding rainfords is a decent choice for a small tank, two spot gobies rarely live long in captivity.