As I understand the situation and I'm no discus expert having never owned any, then I'd personally give some salt a try, if its an infection then salt wont do any harm and might sort your fish out. But as i say I'm thinking of this for my fish, and I know discus are very water param sensative, but eitherway if your worried then add a little salt and watch your fish carefully. For most fish 1table spoon of salt per 5 gallons is a good ratio, but like I say you may want to get some more advice on here before adding any as I'm not sure of the effecvt on salt and discus.