I have kept fresh water Rays for many years. They are wonderfully intelligent animals, quite hardy and easy to breed. One of the issues with rays is of course space and pristine water quality. Rays eat a tremendous amount of food. This makes maintaining water quality difficult. The best way I have found is a continuous drip system that is constantly changing water in the system. IMO minimum tank size would have a foot print of no less than 4 by 6 feet for a single adult Motoro ray. Here is a link that will show some of the different species of FW rays.
http://www.freshwaterstingray.nl/Frames%20Engels/English.html
I feel that salties should be left in the ocean as the success rate with them is very low.
More later I need to go back to bed. Do your research first but I can tell you that FW rays are well worth the time and money involved in keeping these fascinating animals.