I've seen a large, beautiful tank where rays had been kept with discus for many months, and they all looked pretty happy & healthy to me.
But I've also heard a couple of horror stories where adult discus picked the eyes right out of the rays' heads - not sure I can believe that, but I've heard the same thing from several people. Who knows - I guess if one must try, one will & see how it goes.
Friend of mine keeps a 220 with a ray and several discus. All get along happily. She hand feeds the ray also.
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