Most Marine based sands aren't adequate either.. They are crushed coral or fine aragonite and will have the same effect. I asked him once already to check the color of the underbelly of the ray, but got no response..
Also asked to see what the difference between your tap water pH and your tank water pH.. You acted surprised when your pH was 8, and you're using marine sand. This shows me that the pH of your tap water is typically lower then 8, and the marine calcium-based sand is what buffers the pH back up to 8.
Everytime you do a major water change you could be causing a sudden shift in pH, and that can be bad for your ray.
Hopefully someone listens to me this time ...