Did you get any live plants for the tank? Because a lot of the plants carry snails on them.

Sorry, I'm not sure what you can do to get rid of them. Whatever kind of snail that is multiplys a lot. If you did buy real plants, you might want to get rid of them because there could be eggs on them. I think when my old tank had that problem, my dad did a full water change and just bought new gravel and fake plants. But if you don't want to do the full water change, try looking online and see if there is anything else you can do.