So that there is no confusion that end up in really sad oops, here's a quick heads up. If you do decide to go pick up something at home depot, make sure you aren't getting a water softener. A water softener uses salt which will end up killing your fish, while a reverse osmosis system removes the minerals and other contanimnants though an extremely fine membrane. You can often buy R/O water from you local fish store, and if you don't have a large tank, this maybe how you want to do it for a while. How big is your tank, and what are the inhabitants?
I know people use other ways to soften water through peat moss, Indian Almond leaves, etc., but I'm not that familiar with these methods. Hopefully, a bump to the top will get someone's attention who knows how that works.