Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
You can test the GH and KH etc through the week. If the plants are using a lot of Ca and Mg, and you find the GH dropping for example, then it is OK to top off with water that adds back just the amount of minerals you need.
However, in my low tech tanks the plants did not use the minerals that fast, so top off with rain water was helpful after using tap water through the summer. TDS climbs through the summer unless I do a really big water change.