I think it will work for a while, but eventually something has to give. I had motoros and a leopoldi x pearl with an assortment of large tankmates for about two years, then it went downhill fast. First it began with 6 burundi frontosa about 6 inches each they had grown up with in a 24 hr period, saved the rest of the colony once I set another tank up. Then a 6 inch flagfin and numerous similar sized tinfoil barbs, all fast swimmers but everyone has to sleep sometimes. I moved out all the smaller tankmates when the rays got to 12 inches and left only fish that should be off the menu. Over the next year and a half they darted a 17 inch tigrinus, a 17 inch broko peacock bass, and a royal clown knife over 24 inches. The got a little to close during feeding time and took a barb from a defensive ray. The tig took almost a month to die, swimming in clockwise circles and not able to stay balanced, clearly a neurological problem due to toxin. The bass and the knife both did the exact same thing and died within 18 hours of being darted.
Point is, try it for a while but be ready to move the discus after the first attack. Once they get the taste its over. Only thing I have had long term without issue is a silver arowana. They were together until the aro hit the 30 inch mark then it was time to go. He went to a 500 gallon.
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