Hard to tell for certain, but looks very much like an amphipod, aka a scud. A quick google search on 'em should give you some images to check against to be certain.
Harmless, but most folks dislike 'em. Pretty much impossible to kill off once they've got a good population going, though most fish will happily munch on them given a chance.
Last edited by Knotyoureality; 10-22-2012 at 05:28 AM.