This is an issue I have always had, with tap water that is near-zero GH and KH and the pH used to be well below 6. I had a Tetra kit that went down to 5, but it was likely below that. The water board is now adding ash to raise the pH to 7.
With weekly 50% water changes my tanks are now running between 6.4 and 6.8 and that is fine with me. However, hard minerals are lacking and this is more of an issue for plants than soft water fish.
If you intend plants in your Discus
tank, you will want to increase the GH to around 5 dGH. Equilibrium (a Seachem product) works well. You can use dolomite/aragonite/crushed coral but in order to get the GH up the pH will be at 8 or higher. Equilibrium is better since it does not affect pH (or KH).
Or you can do a truly authentic Discus
habitat, with sand, lots of branches and chunks of bogwood, and few or no substrate-planted plants, but lots of floating plants.