The trace ammount of ammonia is a clear indication that your tank is underfiltered. To keep discus, you will have to be serious about quality filtration. I don't mean to bash you choice in filters, but I consider Whispers to be the bottom of the heap of all HOB filters. For a long time they had the absolute least amount of bio-media of any filter. I am uncertain if they have since improved their product line. I am no longer a fan of HOBs anyway because of the inadequate amount of bio-material. Your fish will be much better off if you added a second filter, preferably with a bio-wheel. The bio-pads from Whispers will fit in Penguin filters to give you double the bio area. Then when you clean the filters, alternate which filter gets cleaned to minimize bacteria kill off. The best thing you can do for your fish is to move up a level to sumps or canisters.
The filter isn't the culprit of the fish's deaths. For the ammonia to be an influence, the pH would have to be sky high. What is the pH by the way?
The fish's deaths were related to them breathing hard at the surface. It was either a lack of oxygen or gill damage. Low oxygen would be a perpetual problem, so it is most likely gill damage. It is most likely related to your water change method. So...what is your water change method?