Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
KH buffers the pH.
When KH is low, the pH can vary.
Other things in the water will dictate what the pH is. For example, organic acids from peat moss will make the water more acidic. CO2 will make the water more acidic.
Materials added by the water company can raise the pH. Usually the water company does not add carbonates, but adds other materials (such as sodium hydroxide) that will raise the pH.
pH is easy to change when the KH is low.
When the KH is higher the pH is more stable, and tends to be higher.
Stable is good if it is where you want it, higher KH makes it hard to change the pH.
For your Pencil Fish, I would see what happens when you treat some tap water with peat moss.
Put half a cup to a cup of peat moss in a 5 gallon bucket with a couple of gallons of water. Test the water before you put it in there (GH, KH, pH) then stir the water as often as you can, or run a small pump, and test the water daily for a day or two. If the test results look better, and you want to do the work, then it would mean pre-treating the water before each water change.
I run the water into a garbage can with a knee-hi stocking of peat moss and circulate the water for a day or so for my softest water fish. The peat moss lasts for several barrels of water, though the treatment takes longer as the peat moss gets old.