Well, there's really two things that could be causing the deaths, and it's probably a mix of the two factors: stress and disease. I'd go ahead and add some salt at 1 tbs. per 10 gallons at watch them closely. You say you've kept discus for awhile so that should help the diagnosis. Any white, string feces? Any strange behaviors? Are they eating? Just tell us anything you can. Did you put these fish through quaranteen first? Do you have a spare tank lying around that you could put these fish in that doesn't have plants and gravel? Good luck! We're all here for ya.
Brad