You can use distilled water in a pinch but I recommend going with RO/DI water. You can buy a machine that will let you make your own or buy it form your LFS.
Use RO/DI, distilled or in a pinch RO only water with no minerals added for taste. Many RO waters in the store are inteneded for drinking so they remineralize it after treatment to make it taste resfreshing.
The idea is you want to tart with 100% pure water so your salt mix provides everything you need and in the correct amounts. When you use spring water it already has high minerals in most cases so your levels will be off.