As long as your pond doesn't freeze solid they should be okay. Can you get Japanese Trapdoor Snails? They are awesome! They are black so they likely would go unnoticed by the ducks. They don't lay eggs and overrun your pond with offspring. They give birth to live young. I've watched it and it was so cool. I actually had a fish tank with just snails because me and the kids liked them so much. I ordered mine off ebay and at least half of them were female and gave birth within a week of getting them.