the only time those little "pits" of erosion are an issue is if they are developing into pinholes that go all the way through. The snail will usually seal any small holes/fissures/etc during normal growth. All of my snails have some small degree of erosion on the tightest part of spiral. This is probably a cumulative effect of time. I would not be too concerned as you are dosing calcium and feeding them appropriately.