I'm sorry but I have a different opinion about this whole situation. I don't think your ray attacked that discus, rays are not predators/hunter fr. what I read; your discus was probably half dead, laying on the bottom that 's when the ray pick it up. If you look at the ray mouth, it's way under neath the body, and it doesn't have sharp teeth, their teeth are more for grinding, not biting/cutting. If you ever fish for rays, they tend to lay on top of the bait, eat it, swallow, then swim away, the hook is always deep down intheir stomach; rays just don't have that predator/hunter behavior but more of a scavenger/clean up type of fish. OT , halibuts has the similar type kind of body ( flat with mouth underneath), but are pure hunter/stalker. Just my opinion ofcourse.