DEATH TO THE SNAILS! they are a pain - probably had a hitchhiker on one of the plants.

you can try leaving a piece of veggie, like cucumber of zucchini on a shallow dish to draw them in at night. salt can do some damage to them, but also to your corys unfortunately. I think coppersafe can be effective against them, but the other fish & plants could be affected.. i'd try to draw them with veggie slices, remove them as you can and hopefully you can turn the tide without drastic measures. if your tank could hold a yoyo loach, a number of loaches love to eat snails. puffers do to, but won't get along with your fish.