My tap water is usually around pH 8 unmodified. If I use that, with just Amquel, the pH drops off to about neutral over a few weeks with fish. I recently started using DIY CO2 and the pH seems to have stabilized at 6.0 - 6.2 depending on the time of day. I had the bright idea to use Kent Neutral Controller 7.0 in an effort to raise the pH a little. After adding 5 teaspoons over the course of an hour, I realized it is loaded with phosphates. In that lies the sin.....
How would I best stabilize or buffer the pH in the future, assuming I do a radical water change? I have not yet measured the KH or GH or dissolved CO2.
The CO2 test kit is on the way, and I guess the pH/KH relationship is now not too valid, due to the phosphates.