Pond snail. I would get rid of it as they can start a snail invasion...
Bladder snail, to be precise. If there's nothing in the tank except for a filter, be prepared to be overrun. Some got into my main tank, but thankfully the eggs got eaten by some hungry fish.
Pond snail. Bladder snails have shells that whorl to the left. Pond snails also get pretty large. I hear that bladder snails reproduce much faster than pond snails though. Pond snails also have triangular shaped antennas, where bladder snails are very thin.