Tap water weigh more than מי גשמים , and the מי גשמים floats on the top.
The more the Hashuka is opened, the original מי גשמים comes into the Mikva,
and the Mikva tap water goes into the מי גשמים .
Many people are repeating Hashuka daily, not knowing it was done.
Once its marked on chart for that day, it avoids another one.