congraulations! you have pond snails! don't worry they aren't that bad, yet, of course. they probably came in with some live food, or on a plant you planted in your tank. they aren't really bad, in fact, they eat algae, old fish food, and fish waste, helping keep your tank squeaky clean. of course, you can have too much of a good thing, be cause they can do what is know as asexual reproduction, which is an adaptation set on by a lack of males of one species. all of your type of pond snail are female, and they can automatically produce fertile eggs without having to get it on with a male snail, so just one can start a new colony. they can get to be pests, because like I said, they can literally multiply and overrun your tank. if you get too many, and want to get rid of some, just put a large, blanced, piece of lettuce in your tank and the next day, smiply dispose of the leaf, along with the snails that have come to feed on it.