They're not dangerous to the fish.
My guess is that they're pond snails. They probably hitchhiked into your tank from any live plants you recently introduced.
There are a few fish that will eat them, but are usually not compatible with most tanks. The clown loach is one, but needs a tank 75g or larger. Dwarf puffer fish will also, but those are a very aggressive fish.
The only other thing that will eat pond snails is an assassin snail. Those aren't easy to find at the different LFS in my area, so they could be difficult to get.
Something you can do instead of trying to get an animal that eats them is the lettuce trick. Place a piece of lettuce on the bottom of your tank overnight. In the morning, before the lights are on, take out the lettuce and any snails that are on it. Repeat as necessary until they're all gone.