pond snails, They are hitch hikers on a lot of plants and usually in "egg-form" before even planting. So you don't notice any snails till 2 weeks. Assassin snails will take care of them and can be sold afterwards once you remove the problem.
I personally like keeping pond snails for another cleaner of the tank but also have an assassin snail that keeps them in check with mating. trust me, if you accidentally overfeed your aquarium - they multiple like rabbits! placing their eggs on the surface of the water glass the first week then carrying them back down and attaching them to a plant or hardscape to hatch.
Only problem with assassin snails or killing snails in general is their fricken shell doesn't decompose!