I'd love to comment in full but I'm having problems with my discus tank at the mo.
There are many products to soften tap water, if you want pure soft water then get an ro machine. Most people on here will disagree that you need soft water/ro to keep discus, search the forums and make your own mind up. There are many different ways of thinking that people on here keep there discus in. Wlydfya, one of the mods has massive ph swings everyday with no problems, others like myself have struggled to keep them at times in the text book(old fashioned) discus conditions, again search, read, search and read some more. Most tanks bred discus will be fine with anything they are used to.
The best course of action by far is to get your discus from a very good breeder/dealer and use what their discus are used to and go with that. I know at lot of people keep discus in a variety of water conditions and everyone who has them says their way is fine, simply because it works for them. If you get them from one good outlet and keep them in the same environment you will have less problems in the long term