dont worry . your tap water will be fine . they are not as sensitive/delicate as they are made out to be . as hamfist says they wont like high nitrates , but can certainly handle levels higher than 10ppm no problems. just keep your water clean , do plenty of regular waterchanges and your discus should be absolutely fine .
I can second this with my own experience concerning discus and nitrates. In the fall unusually heavy and prolonged rains caused our tap levels to spike above legal limits, to 50ppm or so. When I became aware of this I was extremely concerned, but the discus by that point had already been living with it for perhaps a week or two. During the period of elevated nitrates (ended up being a over a month before the rains abated and it normalized) they suffered no ill effects whatsoever, continuing to eat like pigs and grow well. In fact, the only thing that stressed them out was me upping the water changes to attempt to keep it from getting any higher. This is kind of an extreme case and was a one-off thing- we got more than our annual rainfall in a matter of weeks, which is unheard of here- but it's a good example of their resilience.
After researching it of course I discovered that nitrates actually have to be pretty darn high to harm fish (even those deemed "sensitive"), but it still had me stressed even if it didn't bother them! They're a lot hardier than people give them credit for.
