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Filling a Mikvah from the Faucet

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 a mikveh from a faucet. There is a copy of the book on Hebrew Books.

So why can't we fill a mikvah from a faucet?