Though it may make a difference to you whether you drink purified water or tap water, I doubt you think about these differences in regards to your water fountain. After all, the water is just for show, and no one is drinking it. But, as far as cleanliness and the longevity of your fountain goes, you’ll be better off putting purified water in your backyard fountain or indoor fountain.
Tap water has a much higher concentration of calcium than purified water. That means, especially if you’re in an area with hard water, you will really struggle with white scale. As the water evaporates, the calcium deposits stay, creating a white ring around the basin of your fountain, which is called white scale. If you have a glass or metal fountain, this is especially unsightly. The white scale can be washed off, but it takes vigorous scrubbing and will keep recurring because of the water. By using purified or bottled water for your fountain, it will stay cleaner and clearer.
If you are an animal lover or bird watcher, the water which you put in your bird bath or water feature matters. If you are using well water, the water could be contaminated by lead or other hazardous chemicals which have leached into the supply, which is not good for the animals drinking out of your feature. Tap water can have high levels of chlorine, or if it has a metallic taste, may be contaminated by corroding pipes in your house. Also not good for the wildlife, if you can help it.
Though it’s very important to consider what you put in your body, it’s worth consideration what kind of water you put in your fountain or feature.