They aren't mystery snails. Mystery snails reproduce differently, and any egg sacks would be easily disposed of, since they are large and above the water line.
Clean the tank more often and better, making certain that you clean the substrate very well, and reduce feedings. Snails problems only occur in tanks that are overfed and undercleaned. If you don't already clean the tank weekly, start now, and do small water changes initially so there aren't any sudden changes for the fish.
Adding more fish, even ones that eat snails, is a poor option since it means you end up having MORE fish to feed, producing even MORE waste.