Here's a photo I just found of my Otorongo Ray (Potomotrygon castexi) in his new home at The New England Aquarium. They keep him the "Primitive Fish" display with a large lungfish, alligator gar and an arowana.
I had to move and couldn't get the tank and the fish moved at the same time. Freshwater rays are very sensitive and they wouldn't have survived the move. Had no choice but to donate them. I also gave them a P.motoro that's in their large Amazon display. Oddly, I had to teach them how to keep them alive. They'd killed every previous FW ray they'd ever got in short order.
I fed mine live shiners, nightcrawlers, live crayfish and fresh frozen tiger shrimp supplemented with the same vitamin and calcium powders I used for the reptiles. They lived in a custom 4' x 6' tank I designed specifically for rays.
I donated them in late 2001 or early 2002. This photo of my old P.castexi was taken around the time you visited. I should really take a trip soon to see if they're both still there and get some new photos.