Medications you should have on hand (and not wait till the fish is sick) when keeping discus are Furan2, Metronidazole, Prazi, rock salt and epsom salt. The rest can all go in the bin because they're a waste of time and money.
Do at least 75% waterchange (with treated water of same temp) before treating with Furan2 and follow instruction on packing. Make sure you do the full treatment.
Tell the fishshop what you want and not let them tell you what you need. If they don't have it go to another shop.
Good Luck,
ron