I had a phosphate issue when I was using tap water. It did not seem to bother the snails at all. As Akronviper stated, at least they feasted on the algae the tap water created.
I did start using the ro/di and it has made a difference. As akronviper also stated, it took more time to clean up the mess the tap water made...algae, phosphates, nitrates, cyano. I thought it was just easier to use the tap water in the beginning....it certainly wasn't!