I wouldn't sweat it. I'll bet most ponds in the midwest have snails. I know mine do....and we swim in them, and eat the fish from them. You have to realize that despite all of our hands-on management, we're still dealing with ponds after all....I believe there will always be surprises, exceptions, disappointments, triumphs, tribulations, and setbacks.....that's just how it is. Unless you place it under a giant plexiglass dome, you're going to have to contend with stuff like this. Seldom, if ever, are such events apocalyptic in nature.
I would consider said events, (and snails in general), the norm, rather than the exception.
