Reverse Osmosis Water. Its is basically tap water filtered 5 times for the most pure water you can buy. Your fish will love you for this.....
Our Tap Water has tons of micro stuff in it that can add bad elements into an aquarium. So if you start out with a pure water, and keep up with cleaning it, your fish will literally hop out of the tank, give you a high five, and hop back in. (Not literally but you get the point)