Having had aquariums in the past, their eggs usually hitch a ride on a pet store plant. If you wake up one day with tiny snails that slowly grow in size and population, most likely eggs got in or are getting in so consider treating plants in a small aquarium before putting them in and avoid anything that can harm frogs, fish, etc.. If the snails just appeared one day and were large, those are probably local residents.
I've taken a long blade of grass (like 18" long) and gotten a snail to stick to it, then dangled it in front of different resident green frogs that inhabit my pond. I usually sit on a large rock near shore, and 2-4 females are usually at shore's edge on either side. They will eat a dangling snail as the grass blade bobs in front of them. Kinda funny as they let out a strange croak as they go for it. Websites say they eat them, but dunno how they pass the shells. I remember having zebra fish in an aquarium as a kid, and I'd take a long spoon and crush the snails on the tank sides, and a zebra fish would eat it. Unfortunately, it could NOT pass the shell bits and would rupture their stomachs weeks later.
Anyhow, maybe consider a catfish-type of fish known for eating them. Other fish will have to actually see it, then decide if it actually wants to eat it. If they are large, you'll need a larger fish or possibly the snails are too large for most of what will be in a pond setting.
