Malaysian trumpet snails (MTS, Melanoides tuberculata ) are good for keeping your sand turned over. MTS will eat also eat algae but not to the extent that they clean the glass (see left). If you MTS are all up near the water surface this may mean you have a water quality issue! So they can a useful water quality indicator in a cycling tank.
They reproduce at a moderate level, but don't normally become a problem unless you are over-feeding. You can often get a few for free from a fish store or thrown into an online order.