I'll take a look at the sump, but everything in there has worked out so far. Water comes down overflow into a filter sock, water moves over bio-media, which is in the same chamber as the two heaters, then through one final piece of "sponge". I forget the actual name of it, but it's black and more like stiff plastic filament that was extruded into a block shape. From there, it's through the return pump back to the aquarium. The sump gets vacuumed every month, in case debris finds its way past the filter sock. I'll definitely keep that in mind.
I'll keep an eye on the other 4, but 2 are preoccupied with their current clutch and the other two are just hanging out. They all come to the front every time I go to check on them (they would eat 10x/day if I let them). So far, they all look normal.
I've never heard of adult discus getting beat up that bad either. There is breeding activity, and there are confirmed pairs so it's a possibility that they're wanting their own space. The two in the hospital were spawning every 10 days for awhile and then the female switched mates (maybe the male got depressed?).
I'm keeping an eye out and checking on the hospital tank every 20 minutes or so just to see if there's any change to them. Luckily I am working from home for the forseeable future due to COVID-19, so I've got even more time to spend with them. Perhaps I'll start trying to talk the wife into another tank if it turns out to be bullying due to spawning.