Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
There is not any one perfect reading for GH, KH or pH. Fish, shrimp, snails and plants come from a wide range of natural waters with all sorts of mineral levels.
Make the GH (not KH) suit the fish.
Make the pH right by controlling the KH. In general fish that like low GH also prefer low pH, and this means low KH. Hard water (that is, high GH) critters usually prefer higher pH, and to make the pH higher, raise the KH.
I have quite a few tanks with no KH, and the pH is very stable. (very low, but that is stable).
Also, pH swings are not deadly. CO2 will make the pH change over a 24 hour cycle. Fish are fine with this.
Changing mineral levels can be deadly, though.