Folks, Comets, Koi, even Fantails will eat snails, tadpoles, smaller fish, etc. They are omnivores, meaning the eat plant and animal food products. They are all carp and the all have a lot stronger suction power than most people think. Even small ones can suck the insides out of a snail shell very quickly. If you want to keep snails I recommend keeping the adults in a stream bed, or better yet a plant box. The young snails can then make their way to the pond and if they get eaten it will be only a few at the time and you should be able to maintain a population with out overload and without extinction. But beware, snails are hard to monitor, They do take up oxygen and produce waste just like fish! If you happen to have a water condition change your not aware of and several snails die at once you may not know it. This can very easily start a worse effect on the pond as by the time you notice the snails are dying/overloading your filter bed, your fish can be in serious trouble.
