I see a few issues here for a discus tank.1st the tank is way over stocked.I would take out the discus and the severums if it was my tank;maybe leave in two severums.Your tank parameters are off;nitrates should not be zero even in a high tech tank,which I assume you have because of the CO2.Of course your also adding ferts to make it a high tech.With that percentage a WC your leaving a lot of **** behind.You'll also need to clean the substrate if your keeping discus,due to all the anaerobic bacteria in the substrate;discus are prone to Hex.Thus poor water quality (for discus).The other fish in your tank will live just fine with your WC schedule,but things are different for the discus;they'll need more frequent and larger WC's.