Originally Posted by Byron
I would respectfully suggest there is more behind these numbers. The GH and KH in an aquarium will remain basically the same as the source water (assuming weekly partial water changes) unless it is acted upon by some substance.
Am I allowed to respectfully disagree with you.
Have you considered and care to comment on this equation:
just before water change value=value in replacement water+(changes between water changes)/(fraction of water change)
Which assumes that things are constant with a constant change. And that sufficient water changes have been done so that the before water change values are the same.
KH is very much tied to the conditions in the tank as well as whatever is in the replacment water. Through interacting with calcium, the forward aerobic bacteria nitrogen cycle, plant action.
So the use of water changes will limit but not prevent deviations from whatever is in the replacement water. In my experience peat moss prevents the build up of both gh and kh. Which resulted in a kh of 4 degrees and a gh of 9 degrees for 2-3 years (before the tank tear down). But after a few years in a plain sand trank gh was over 35 degrees and KH over20.
My point as always is that the tank conditions themselves have a more important effect on tank conditions then partial water changes. That is unless you do something like 100%/day water changes. Which I consider more an open system then closed.
Still just my .02
ps hope that's too too confusing to OP.