Welcome Bungalow.
I would not use the water butt water in place of regular tap water. If you have fish that have a reputation for liking very soft water, you might get away with mixing rain water as 2/3 of the total mix but not much more than that. All fish require some amount of minerals in their water to thrive and if you go with water that has no "contaminants" like iron, sodium or calcium, you will quickly have some very sick fish. Water control for a fresh water fish is a delicate balance. Freshwater fish urinate constantly just to keep from having too much water accumulate in their bodies. If you have water that is too "pure" for the particular fish, they simply can't keep up. The end result, in terms of water, is much like what happens to your hands if you leave them submerged too long in fresh water.