I had snails come in on some water plants in the pond in SoCal and by the end of the summer they had completely clogged the pump impeller. We had to take the whole thing apart and there was a nasty mess of ground snail in there. It was the really small ones that got sucked into the pump, a sequence 750 external. Whenever I'm there I go out at night and pick as many as I can find. Finally, I don't find many in the pond but they are all throughout the skippy. Hmmm, maybe the fish got big enough to eat them and it wasn't all my nighttime flashlight snail picking. I think they are a pest, perhaps especially in areas that don't freeze.