When exactly, does a Zavah become Tahor?
Moshav Zekenim citing the Rambam: All that the Torah obligates them to immerse, they may immerse during the day, except for a Nidah or Yoledes.
Since the Torah writes "Achar Tit'har" ('after the counting she will become Tahor') and she counts already at the beginning of the seventh day (Rashi, Ibid).
What is the source that a Zavah must immerse to become Tahor?
Ramban (11), Moshav Zekenim (2): A Zavah is like a Zav, who becomes Tahor through Tevilah in running water. We should have obligated Mayim Chayim for a Zav; "Achar Tithar" teaches that she becomes Tehorah like other Temei'im in the Torah, who immerse [even in a Mikveh]; they do not need Mayim Chayim.