Rays are tough to keep for a few reasons. Rays need a lot of room to move. Even the smallest rays won't be able to have the room they need to swim in a 50. The stress would be too much. Small bluespot rays, which stay pretty small, need unobstructed swimming space. A minimum of 300 gallons, 7 feet of space to swim with no live rock in the way. The second problem is feeding. Many rays just don't eat in captivity. I have thought of getting one several times, but they just aren't feasible for me. Good luck.
-Doc