I run my water through only a sediment filter and a couple carbon blocks. I am on city water and it comes out of the tap a little over 7.0 pH. I have not found a reason to worry about changing it. I have a large RO/DI unit that I use for my saltwater reef tanks but not on the rays at all. If I had high pH water (8.0+) I would for sure utilize a softner.
Anyway, the rays went from soft to hard water in one day and are doing fine...so far.