Your discus is suffering from internal heximita and most likely other protozoan flagellates. I'm guessing that the feces are white/clear and stringy when you happen to notice it. This particular fish is extremely thin and I'm honestly surprised that it is taking food at all (is it possibly "eating" only to spit it back out with you not noticing?). The fish is an adult, which I can tell from the size of the eyes. Adults do perfectly fine at 80-82 degrees...this is where 90% of experienced discus keepers set the temperature of their tanks. 86 is more commonly seen in fry-3.5"+ sized fish. After that, temps are decreased. In the wild discus can live in water up to as much as 94 degrees believe it or not. Your cardinals would most likely be fine for the higher temps for a week or two, however I recommend treating in a smaller QT tank.
For treatment, you should administer metronidazole at a rate of 500mg/10g water. For the metro to be effective however, you must have the temperature of the tank set to 90-92. Dose every day for 7-10 days straight, changing half of the water every other day with warm water to keep the temp at 90-92 degrees. In addition, you will want to get some frozen bloodworms (if the fish is in fact eating) and crush up some metronidazole tablets and mix about one teaspoon of powder/crushed tablets per half pound of thawed food. Let the food absorb the metronidazole, and then feed some of this food exclusively to your fish for two weeks. This is by far the most effective method of treating internal heximita. I am recommending both methods of treatment at the same time.
By looking at the fish and from numerous personal experiences with discus, I would have to say that I believe this fish should be put down as the chances of recovery do not look favorable at all. If the fish is still eating though, then there is always a chance, but don't expect any sort of miracle here as the fish may be too far gone.