I use tap water.
Not to many limitations with the smaller rays like histrix or retics. Geos would be fine with them, I have seen that done alot. You could do clowns loaches, catfish, aros, and stuff like that. Most upper level schooling fish are fine. Really anything the rays can't fit in their mouth, or what wont pick on the ray.
The only rays I have had that kill tankmates are my motoros, well my male motoro, he kills pretty much anything.
My retic on the other hand was so placid. I had it with a dojo loach that it could have sucked it up like a worm but left it alone, plus smaller tetras that it could eat no problem. Just depends on the individual ray really, and the what type you have.