So I cleaned my pond today..?

Question:and yes I know, it should have been done in the spring but I was too lazy then. As Im dumping out the water, I see little tiny rock looking things that are moving on their own. SNAILS. what the hell? how do these things just appear? I heard they multiply like crazy and are hard to get rid of. I made sure none of them were in the pond or alive when I refilled it but I know that since they just materialized before they will most likely show up again. When they do, how do I get rid of them? or are they beneficial? which then I wont kill them.

Answers:
The snails appear because the pond was dirty. They are good for your pond because they help keep it clean.Snails will eat algae that forms in your pond.I would not kill them,they are natures way of keeping your pond clean.