this pH thing is a very hot topic. years ago it was you had to get your water as close as possible to the discus's wild parameters. lots of discus were lost although i would never touch them then, believing they were very hard to keep alive. although things have improved and i now keep them, although i have only had mine for about four months, i keep mine as the old books use to tell us. i keep mine in ro water remineralized with either kent ro right or a liquid fert for plants as there is enough minerals/metals in this? and its beneficial for my plants. my pH is 5.8. KH and GH i have never tested for. my rams and apistos breed in the same tank, all my fish are thriving, thats good enough for me.
now for the hot bit.
i don't buy into this match your water's params to the water that your breeder/fishkeepers use. does this include the discus seller, who has obtained his discus from a breeder for re-sale? the breeders have to use ro water unless they have soft acidic water out of their tap other wise the discus will not breed/fertilize/hatch. so for the first six weeks of their lives the discus babies are kept in acidic, low GH water. when the baby discus are old enough to be taken from the parents to be grown as quickly as possible, and to do this in "not the best conditions" ie, alkaline, hardish tapwater lots of them do 100% water changes every day to compensate and keep the nitrates down to as near to nil as possible. then you get your beloved discus home and now have you got to match the breeders water? meaning you have got to put something into the water, possibly altering your water that might be nearer what is best for them before you start to alter it? why not alter it the other way which would be even better? breeders haven't got the time or the capabilities to do it better on such as large a scale as they would need so they change the goal post's to suit themselves. they sell more fish and ease peoples fears. when you get your fish home are you going to do 100% water changes everyday? no. things will look good, even easy. you believe every thing you are told. even get relaxed. and then the nitrates start to build with your confidence that they are an easy fish to look after (anything over 30PPM) and suddenly big trouble. they are open to all sorts of discomforts accelerated by not the best water. which i believe they do need. lots of people will disagree with all this and a very high percentage of those will be breeders.