I had the exact same thing happen to me I had purchased plants from a local fs and these little snails had hitched a ride straight into my tank and made themselves very comfortable. They look like pond snails (Lymnaeidae). I had a very difficult time getting rid of these snails without introducing new fish or chemicals I literally had to strip my tank bare and it still took me months to eliminate them all. They breed extremely quickly, their eggs are laid below water level and are very hard to spot because they are a clear gel laid on just about anything such as the glass, decor, and plants etc. If you do decide you don't want to keep the snails I would act promptly because they can get out of control quickly and are a pain in the behind to eliminate. Hope this might help...Good luck.