I was switching my filter socks along with my water change tonight and when I threw the old ones in the bucket this small shrimplike/wormlike thing started swimming around. Please excuse the poor pic as this thing was very hard to focus on. It is about .5 - .75cm long and mostly clear. It has some antennae or something on its head. In the image it is curled up, but when it swims it straightens out and swims almost like a shrimp. The tank has been setup for roughly 6 months.