they pretty much eat everything including most organic stuff in your substrate, your decaying plants, driftwood, leaves. so you are overfeeding them and just don't know it
once they get really established and get everywhere (substrate, filter, underside of your lid) you can't get rid of them except w/o a predator or a chemical means which is not advisable.
best thing to have done was to remove them from the start. planted tanks w/o snails do exist. i am sure you can get there if you want.