If you venture into the hobby of Discus, there is little doubt that at some stage you will want to have a go at breeding them. Breeding will happen as a matter of course in an aquarium with adult Discus in it. Problems will arise in a tank that has more than just the parent fish present, so it is best that having identified a pair of fish that you put them into a separate aquarium. The size of the breeding cube not too important, but in my opinion, 80 litres is about right; you will of course need a mature filter, a heater and a good deal of luck.
Breeding Discus has become easier in recent years, but the downside of this is an explosion of genetically poor fish. Breeding a strain of Discus that will breed true is a time consuming skilful task. That said nobody should be put off from trying to breed a few Discus from "what they have" because the whole procedure is a wonderful experience and one that hardened breeders never tire of.
People like me can write as much as they like about breeding but the best way to learn is to have a go. Once the fish are settled they will display to each other and clean the chosen spawning site. This can be any vertical surface, but usually on a "spawning cone". They will then take it in turns to guard the eggs, gently fanning them and ensuring a flow of water over them, until after about 60 to 70 hours they hatch. At this point we refer to the fry as "wrigglers" They cling to the breeding site for a couple of days until they become "free swimming" The fry will seek out the parents and cling to their flanks.
The parent fish will turn dark at this point as they produce mucus for their offspring to feed from. This goes on for about a week, the fry grow very quickly. It is essential at this point to hatch out plenty of baby brine shrimp and offer it to the fry several times each day. I personally remove the parents when the fry are the size of a small finger nail (14 days) they are then fed on baby brine shrimp and beef heart mix.
The above is a very simplified version of how to breed Discus, there are of course many problems to overcome, such as egg eating, disappearing fry, mal formed fry and so on. It is not easy, it can be very frustrating, but when successful there is no better feeling.