I generally agree that it's best to avoid water mods if at all possible. That being said, there are couple of things to note. First, as said above, most fish can tolerate water that is different than that of their native waters. Second, stable pH is usually more important than 'perfect' pH. Finally, lowering pH and GH is much, much more difficult than raising it.
What is the GH, KH and pH of your tap water after it has sat out overnight? That would give us the info we need to start to answer your questions more specifically.
Jim