Okay been reading the threads, rly 50/50. I have a 270 (6' x 3'), 3x BD rays (10", 10", & 5"). 6x 5-6" discus. Good combo? My fears are the rays either stressing out the discus or just not letting the discus eat.
i am currently housing 2 marbles(roughly 6/7 inches) and
10 discus (3-5"),, so far no problems at all... the discus actually
pick the worms off of the rays back. the other day though i did see
a ray literally steal a red wiggler right out of the mouth of one of
my discus. 365 gal tank btw
I've been keeping 11 discus with 2 p14 for the past year. In hindsight I would not keep discus with rays again. I started out with 15 discus and 4 of them died from ill thrift that may or may not be due to the rays. I have 1 discus suffer an eye ulcer from a ray gnawing on it and it subsequently lost that eye. Another one lost a gill cover. Every now and then you'll see some bite marks on the discus or losing part of their fin.
In short, I probably will not keep discus with rays in the future. If your discus is large enough, I find the rays cannot physically kill your discus "in a single sitting". My surviving discus are very healthy and eats very well despite a bite from now and then. And I think they have grown sufficiently in size to not withstand any thing more than a small bite to the fins.
Finally its tough to feed the discus well as they take their time to eat and the rays are pigs. You pretty much have to overfeed the ray each time to ensure the discus have sufficient food. I got away by feeding small discus pellets, but not all the discus eat them and my rays are starting to like the pellets as well.