Plenty of fish will eat the eggs and young snails but fewer will eat the adults, so depending on what fish you've got you might only ever notice a few adults or you might have a huge population boom. They're not really harmful unless you've got a lot of tender live plants, in which case you'll probably want to get rid of them. Unfortunately it's just about impossible to get rid of them completely without totally breaking down the tank, but you can control the population by manually removing snails. You can set "traps" for them by dropping in slices of cucumber or zucchini. After a couple of hours it'll be covered in snails and you can just pull it out.
If the water wisteria you mentioned is the only plant you have in the tank, you shouldn't really have any problems and you can just leave the snails be.
Edit: for a more comprehensive article, check Lupin's Pest Snail FAQ stickied at the top of this subforum.
Last edited by iamntbatman; 01-09-2010 at 12:18 AM..