Discus like to be kings of their castle, Any aggressive, fast moving or large fish can distress them as they are peaceful slow moving fish.
1. Tiger barbs with Discus are a deffinate no, as they are active fast fin nippers and would intimidate the discus, shouldn't be together
2. severums though peaceful are not recommended tankmates and their size will eventually intimidate Discus in your size tank
3. Angels can be bullys especially at feeding time, i have one that is fine with my Discus but had to move other angels out for being bullys so could be a problem if any are aggressive, and too many in there for your size tank.
Also all these fish except Angels require lower temps than Discus
in a 52 gal it is only just big enough for the discus a shoal of small tetras and the BN.
Your water needs to be checked and stocking needs to be sorted otherwise you could lose your Discus, sounds like he's stressed and meds won't help stress.
hope this helps
regards Angel