86 degrees would have been a paradise vacation for the discus!
Though the hotter the water gets the lower the oxygen is in the water versus a cooler temp
thats how sensitive the cardinals are =( I noticed when I had the air pumps off due to switching the power strip light and forgot to turn it back on for one night in the 84 degree tank and basically wiped out all but 2 of my cardinals due to that and everyone else was fine so attributed that to the oxygenation.