In this race, bacteria will always win. Bacteria are present in every place of the planet. It has to be this way because their job is to recycle natural matter. When they find an appropriate environment, such as an aquarium, they start to multiply and feed. In a poorly maintained aquarium, bacteria grows rapidly which can contribute to oxygen-depletion which leads to fish stress.
Stressed fish have a lower pathogen resistance.
Discus in particular have a huge slime surface which could be attacked. In this way bacteria have an open road to internal organs and bloodstream. We control bacterial overgrowth with frequent water changes. If we were raising Goldfish, lowering the temperature to fight bacteria would be no problem. But tropical fish require a steady warm-temperature environment.