Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Quite the contrary- Do not add anything to water used to top off a tank from evaporation.
Here is why:
When water evaporates, just the water leaves the tank. The minerals stay behind, making the water in the tank just a little bit harder.
When you top off with pure water (RO, rain, distilled) this dilutes the minerals back to what the tank was like before the water evaporated.
If you kept topping off with minerals in the water then just water evaporates you would be increasing the minerals each time.
It might not be much, if the water was fairly soft, and a water change would correct it lower, but not all the way back to what it was before the water evaporated.