Hmmm. I'm using RODI remineralized to GH 7, KH 3-4, and snails die typically within 48 hours of introduction to both my tanks running that water. Cannot, for the life of me, figure it out. Had to put all my nerites and MTS in my sons 10 gallon tank running tap water.
One of my thoughts was the magnesium sulfate actually causing a problem because of how it regulates osmotic pressure...or something like that. How much Mg are you adding, and I assume you're adding MgSO4.7H2O(Epsom salt)? I started a post about this actually a month or so ago, a couple people agreed that the Mg might be the issue, but how does one rectify that? Lol. Perhaps a different Mg salt...but obviously you need Mg with the Ca, otherwise you'll have unhappy plants.