I have removed most snails from my 400 gallon acrylic tank due to snails scratching the tank. They are not doing it with their shell but with their mouth. Never did really know who it was so just removed all the snails I caught except the small Stomatella snails.
Tom
The only really nasty scratch I have was from an astrae snail. I saw it after it was six or seven inches long and removed the guilty party. There was an edge of his shell that stuck out at a point. I have had no more problems with the 50-60 snails I still have.
It's hard to figure acrylic. I was worried when I added a tuxedo urchin to my tank. He eats the corraline off the acrylic with nasty looking jaws. Yet he hasn't left a scratch so far. Maybe I've just been lucky.