Could well be a pond snail. Their eggs can get into a lot of things and be brought into the tank. Try putting a leaf of lettuce in the bottom of the tank (just set a rock on it to hold it down) overnight, and seeing what you get in the morning. It'll probably be crawling with them, and you can just take it out and throw the lot away. You might not get rid of all of them, but you can control their numbers.
I'd advise against using a chemical treatment to wipe them out, since even if it works, you'll risk an ammonia spike from all the decaying snails buried in your gravel. Depending on your stocking, a loach or two may help (Beware clown loaches - they're sold as a snail solution, but they get too big for most tanks and need very large housing).
How often do you feed? The snails do have to eat something, so if you end up getting a lot of them, its usually a sign of overfeeding.