Try enrofloxacin, but not in these funny doses.
1000mg per 100l first day
2 day 50% water changer and 500mg enro, and continue to 8 day. 50% water change and 500mg enro.
Aquarium without additional filtration, only glass, heater and aerator. Control the pH (range 6-7 is correct) and adjust if necessary - after adding the drug goes up.
I've been using this method since 2013.
It also works on wild discus