I just noticed little black snails (about the size of a pencil eraser) all
over in my waterfalls. Probably hundreds of them. None in the pond, yet.

Will they hurt anything? If I want to get rid of them, can it be reasonably
done, or should I leave them alone? None in the pond, because my Koi ate
them, or maybe they just haven't gotten there yet?

What's strange is that I started my pond last September, and no signs of
them whatsoever until about two weeks ago (and no new plants either). Where
did they come from? I'm kind of averse to snails, unless they come with a
little fork and lots of garlic butter sauce.

Thanks