Yeah.. Nitrates being at Zero could mean an un-cycled tank.. I'm assuming it would be cycled by now though, you've had the ray for 3 months.. Make sure your test kit isn't out-dated..
The pH shouldn't be a problem, unless it was a sudden shift.. Was your pH like this before you added the black moon sand? and how do you know for sure it is ray safe? I have seen alot of people tell me that the tahitian moon sands are not ray safe, and some say otherwise.. If the rays belly is pink, then I would say it is NOT ray-safe.
What is the pH of your tap water? Everytime you do a major water change you might be causing your pH to dip way down and go back up again from the calcium based sand.. pH swings like this can be stressful and might be the real cause of the problem..