Use 1/2 as much light, drop your temp to 84*, and considering your Bio-load, and feeding you should seriously consider getting a second XP3. Rotate servicing these two filters weekly. Set the spray bars 1" below the waterline directing the current at an appx. 6* angle not quite breaking the surface. Make sure your Co2 is on a timed solenoid to shut off at night.

I also strongly suggest picking up one of the surface skimming Bio-System power filters Power Filter: Bio System Power Filter & Surface Skimmer from Drs. Foster and Smith These are a very simple way to acquire surface skimming which helps to maintain optimun Air/Water interface to maximize o2 exchange.

I realize this sounds like alot, but Discus like higher temperatures, and unfortunately they also tend to thrive on High o2. People also tend to feed them quite heavily and this equals an extremely high nutrient potential, and ultimately Algae if unattended. In your case high heat, limited surface area, high stocking, and heavy feeding is crippling your o2 levels. HTH. Prof M