Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Right out of the tap is fine.
If the water is evaporating then the GH and KH are getting concentrated that way.
If there are things in the container (like sea shells, coral or limestone) then they are adding their minerals to the water. Is this in the tank you are testing the 4 hour and 6 day water?
GH and KH should not change if the water is just sitting in a glass on the kitchen counter. (Unless the water is evaporating). Nowhere for more minerals to come from.
Are you reporting the KH in degrees and the GH in ppm? No need to make the translation. Just add the units so we all know.
API kit: The amount of drops you add = the degrees.