All Discus will live in water prepared by a reverse osmosis filter but much will depend on the water your fish have spent their lives in prior to reaching your tank.
Wild Discus will require soft, slightly acidic water and generally the only way to achieve this is to use a reverse osmosis filter. Some areas of the UK have soft water on tap, but, even then, it would be wise to run it through carbon.
Asian Discus are also raised in soft water, so will appreciate the same water preparation for wild Discus. Some aquarists acclimatise their Asian Discus to UK tapwater, but top breeders advise soft water.
European-bred Discus, however, are quite happy and will even breed in harder water – the reason being they have now been tank-bred over several generations and have adapted to it.