Discus can be kept with certain species of rays with no problems (usually). Retics (cheapest but hardest to get to eat, also one of the smallest) like softer water so they should co-exist perfectly with them.
you chould put some blood parrots in as no one cares if that hybrid crap gets eatern
angles and discus are a flimsy type of chiclid
most mid water fish will end up as ray food sooner or later its just a matter of time
uaru are a lot stronger than a angle or discus
when i was keeping uaru.f it was only a temp thing they are all sold now the rays are bigger
a perfect tank mate for a ray tank are siamese tigers they are proven not to be eatern buy rays as the rays dont like the feel of the sharp dorsal fin when they catch the tiger they let them go