Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I have seen that, and, like you, I associate it with low mineral, low carbonate, acidic water.
You could try adding a bit of cuttlebone to the tank and see if the snails will crawl over it and eat it.
Cuttlebone will slowly dissolve in water, raising the GH and KH a little bit. It is very slow, and weekly water changes will correct this back down to where you want to keep the levels. MIght be just enough to help the snails.