Low temperatures weaken the disucs fishes imune system, making them IME prone to exturnal parasite infection. The main threat is flukes or costiasis, both of which set-in after a stress related problem. Lower temperatures are also reported to make these fish prone to worm or other internal parasite infections. This seams consistant to me, as after a heater failure a few months ago, both internal parasites and flukes came out to play in my tank
As DiscusLova says, ideal rages are 28-31, with most going for 30. When I have issues with my disucs, I cranck the temperature up to about 35, as this is past the point where most parasites can survive, but the Discus tolerate the heats very well IME. They have pulled out of fluke an protozoan infections for me with this treatment alone.
In short, temperature is one of many things that could caurse your discuc issues, so make use it is right if you want to keep them alive, well, healthy and most importantly thriving.
Set-up issues will be the main caurse of problems with discus, as I know as my set-up caursed issues for many months with me untill I nailed the fault
HTH
Rabbut