Discus is one of the most sensitive and difficult to keep freshwater fish. Discus are bred for their colours and patterns like bettas and guppies, but they are way more difficult to breed. Discus are aggresive (discus are cichlids) like bettas (bettas are not cichlids but aggresive anyway) but they don't breed as readily. If you put a male and female discus together, they might not like each other and will never breed! To find potential breeding pairs, you need to observe their behaviour in a group to look for pairs they form themselves.
Discus turn pale easily due to their sensitivity. When they turn pale, quickly check the water for nitrogenous waste levels. When they turn black, they're on the verge of death. If there is white stuff on the black, they almost dead.
Healthy discus should be roundish and not shape like a rugby ball. If not fed enough when young, they will not grow into a nicely shaped adults even if subsequently looked after. Most often, their size will be stunted and will not grow bigger than the size of a fist. There a some cases where stunted discus breed. If looked after properly, the fries does grow up into normal adults.