I really like Joshua's answer! I am a little different in my approach to water though. I have a pond and I have always used water from it for my plants. I have no scientific knowledge on this subject but I have always believed that natural is better. RO systems waist a TON of water and from my understanding strip the water from beneficial micro nutrients (just my understanding). I have installed water purification systems like Aquasana products in  homes for many years that remove impurities naturally without stripping the water of micro nutrients. For my ph up or down I have always gotten them from an aquarium store because I figured if it doesn't kill the fish or amphibians it's probably OK. I'm not saying that I know the right way to do anything but this is what works for me.