Bit of interesting lore.. they don't lay eggs like most snails.. instead, they lay eggs inside an internal brood pouch, then 'birth' live young, fully formed, at the water surface, typically at night. You see them crawling up the glass to do this at night. Baby snails often end up in filters as a result.
They are really quite good algae eaters, and because they like to burrow in the substrate, they aerate it, which can help prevent gas pockets from forming, especially if you tend to overfeed fish.
They have several young at one time, so they do increase in numbers quite fast, but they have relatively little bioload, and are very useful. Don't eat plants, do eat algae and leftovers, and don't harm anything else. Get to about one and one eighth inch long at most, and the tip of their shells often vanishes as they get older, but does not seem to slow them down at all if this happens.
