I read a book by Rabbi David Miller (1869 – 1939) ( he had semicha from Rabbi Isaac Elchanan of Kovno ) , The Secret of the Jew, which says you can fill an empty mikveh straight from a faucet (see pages 366 and 428). There is a copy of the book on Hebrew Books.
I don't think I have ever seen this practiced; as far as I know every Mikvah I've seen is filled entirely by rainwater. Are there opinions that argue on Rabbi Miller? Who are they, and what is the basis of their disagreement?