The KH that would not stop falling
For most of the year the tank sat at a comfortable 4 degrees KH, pH 7.2 in the morning and 7.5 at lights off. Then in August the KH read 3, in September 2, and by the first week of October it was 1 and the pH was dropping to 6.4 overnight and climbing to 7.3 by late afternoon. A swing like that is harder on fish than a steady low number, and two of the tetras started hanging at the surface in the morning.
I assumed the tank was consuming it, because nitrification produces acid and acid eats carbonate, and a tank that has been running two years with a heavier fish load than it started with would do exactly that. But when I finally tested the tap water instead of the tank, the tap came out at 1 degree KH as well. The utility had changed something upstream over the summer, or the source had, and every water change was replacing what little buffer the tank had with water that had none. The tank was not consuming carbonate; I was removing it by the bucket.
The fix was small and boring. I now add a level quarter teaspoon of baking soda per 10 liters of change water, dissolved first in a jug, which brings the fresh water to about 3 degrees KH. I did not try to correct the whole tank at once: three weekly changes brought it from 1 to 3 and the pH swing narrowed to half a degree. The tetras came off the surface after the second week.
- Test the tap, not just the tank, whenever a parameter drifts for more than a month.
- Change the buffer slowly; the fish care about the swing, not the number.
- Write the tap reading down with the date. Mine turns out to vary by season.
I keep a jar of tap water on the windowsill now and test it on the first Sunday of every month. It costs one drop of reagent and has already caught a second drift in the spring.