osteoporoosi: I think your first picture is the Malaysian trumpet snail and the second one is a ramshorn. I don't think I have either of these. It's more like
http://www.bigai.ne.jp/pic_book/data42/r004138.html
So it is the common pond snail.
Still, I'm not panicking. I think a few snails would be a good idea to help keep the substrate clean. I do want to prevent a population exlosion, however, so I might start taking preventative measures, like baiting them with cucumber to try to get rid of a few of them. I could also toss in a few Malaysian trumpet snaiils to compete with them. I've seen them in tanks at the LFS so I know where I can get them from.