If you’re talking about using a water softener, it may be a problem. Water softeners us a process of ion exchange where the calcium ions in the hard water are exchanged for sodium ions. The sodium ions don’t react with soap like the calcium does and don’t leave hard water spots.
I would think the calcium ions would be better for the snakes than sodium.
What Brent is doing with the Reverse Osmosis system is filtering things out and not replacing it with anything.
Bottom line, I would think the more natural condition would be for hard water. If normal is to drink from a mud puddle or a stream, normal would be hard water.