My LFS keeps them in 82 degrees but says they can stand as low as 78 if they are domestic and not wild caught. Years ago, keeping Discus was somewhat of right of passage for freshwater aquarists. Though a difficult breed, their 'shyness', temp and water quality needs has been tempered through domestication. I disagree with the some of aforementioned advice. I do think it's ok you get a fish because it looks cool, provided you do the necessary research, especially since discus are a significant investment. As far as having a tank "just for discus" I'm not sure what that means...monospecies, biotope, planted? I know breeders often use completely empty tanks to breed discus...easier to clean. Some advice I might agree with is not to overstock your discus tank (depending on what "an uncontrolled population of guppies" means) because they do still require good water quality, still retain a measure of shyness and are temperamental eaters, especially when first aquired.
Can't say I agree with much of this, nor would anyone in my view who keeps these fish (Discus) .
They aren't a particularly difficult fish to keep if their needs are met.
Keeping them in lower temps slows down their metabolisim,growth, digestive process,and perhaps could lead to compromised immune system over the long haulThus making them more suceptible to bacterial pathogens and or parasites.
They still require several small feedings a day as juveniles to achieve proper growth, much of which occurs during the first few months and subsequently,, frequent water changes may be in order to keep dissolved organics from polluting the tank.
Would agree that many are able to be kept in moderately hard or alkaline water with exception of those bred and raised in soft water, or wild caught specimens, but spawning success in same may prove difficult if that is ones aim.
Saying these fish will tolerate waters or temperatures that may be at upper or lower end of their comfort level is quite different than fish that are thriving. (opinions vary)
Am aware that many fishes are kept this way but often times,,,NFL (not for long).
Would agree that researching the fishes needs is wise before purchasing the fishes.