My tap water GH is around 15 ,but when I used to change water too often(partial water changes) ,The GH and Kh had climbed in the 20's
Some of the water evaporates but the minerals contained stay in the tank.Add more hard water on top of that and the GH and KH go higher ,each time.
What I do now is top off with RO/DI water ,and also if I make a water change I also dilute tap with RO water.
I plan to measure GH and Kh monthly ,after the WC ,and monitor so that I don't lower them too much.
Hard water sucks@!
Can't come to work today ,Boss......I've got Bolbitis...