I believe that a simple sediment/carbon stop system removes a significant amount of water hardness; a simple RO system will practically eliminate water hardness, but a system like that will cost you over 100 bucks, and from what I've heard stingrays are not fond of water from a reverse osmosis system (due to the lack of important minerals that are in the water.)
In any case, my water comes out sometimes with scales (especially after you move the faucet in the kitchen around, and no I'm not kidding) and my rays have done fine in it. As long as you don't accidentally poison your rays like I did (long story) I don't see any reason why they can't thrive in almost any water that comes out of the tap, given that the chlorine is removed and there are no high levels of lead or mercury or something.
I wasn't always the same and learnt lessons the hard way when I was younger! It's a lot easier doing research now with have the internet and great forums like these, I find some of the best research is going through all the threads and reading peoples personal experience on here, a lot of hours are lost to this forum! I like to be fully clued up nowadays