Indoors, you need to use water that has minerals in it.
Rain water has NO calcium carbonate in it whatsoever, and your plants NEED that to grow. Also, rain picks up contamination in the atmosphere, such as Nitrogen and Sulfur from smog and other fossil fuel combustion byproducts. When that interacts with moisture in the atmosphere, it goes into solution as nitric and sulfuric acid- not good! It will quickly use up the buffering capacity of your soil, which is guess what, calcium carbonate added at the packaging plant, and the pH will suddenly drop.
So if you use rain water, you must add CalMag Plus to it, and then adjust the pH before each watering.
Tap water is almost always better.