Hi Andyking, are we talking domestic or wild discus? Both domestic and wild discus can handle a ph of 8. I have my wilds currently at 8.2. Back to your question and remember your KH figures into this. Try mixing a ratio of 80% tap to 20% RO, age it and then measure. You will need to play with the mix until you get your desired ph. BTW welcome to SimplyDiscus.
Pat