Pond Snails (ie Pest Snails) are hemaphroditic and only need two of their species to create millions of offspring over a few generations. They are prolific and can take over a tank. I crush them when I see them, it's actually part of my job detail in my job at the LFS. They aren't necccessarily harmful but are a major nuiscance.
I couldn't agree more except for the part about them not being harmful. I had such a problem with trumpet snails once that I had to completely redo the tank. Took out the sand and threw everything away just to get rid of them. The harmful part is that once they multiply so fast and there are so many of them that they create an enormous bio-load and they will eat your plants (maybe that's not exactly harmful). When I had my problem, they were driving my nitrates out the roof each week (my nitrates generally don't get above 20ppm even if I go 2 weeks between water changes) but fortunately I was overfiltered so I never had any issues with ammonia or nitrite.
