Salt does not hurt discus in the least. Discus like most of the fish in the Amazon eveloved from salt water fish to begin with. That is why so many of them have such bright colors. Also twice a year the ocean will back flow into the Amazon region putting loads of salt water into the system. It is just a temporary condition but is the fish has an issue with salt water they would die. Having said that I would not use salt on an everyday basis but it should be the first thing you reach for if the fish gets sick. Normally 2 heaping tablespoons of rock salt per 10 gallons of water for 10 days will cure most illinesses. Replace the salt that is lost though water changes. And yes I did say rock salt, don't waster money on aqauirum salt. There is no need.
-john