Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Water companies often add something to the water to maintain an alkaline pH while the water is in the pipes. This may be something with a short life- they do not much care if it breaks down by the time the water gets into your pipes. Water may also have a substantial amount of carbonates. Water quality report may show this as Alkalinity or Carbonate Hardness, aquarium tests are usually labeled KH.
I suspect (but have no way to prove it) that high CEC substrates can remove enough of whatever is added to allow the pH to drop. I KNOW high CEC substrates remove the carbonates (KH).
So, without the pH raising material from water company or the carbonates, 'other things' in the tank can control the pH.
In this case, the organic acids from the driftwood are probably the controlling factor. Many organic processes such as decomposition will also lower the pH. Nitrifying bacteria will also use carbonates, and this can also drop the pH.
Here are a few things you can try, see if you can figure this out. In all cases, test GH, KH, pH several times through the test. Stirring often, or adding a bubbler can make some of these reactions happen a little faster, or at least make sure the sample is well mixed.
1) Tap water in a glass, let it sit out for several days to a week (or shorter, if you add a bubbler).
Test when you first fill the glass, then 12-24 hours later, then daily for as long as you want. This will give you a baseline of how the water all by itself responds to being exposed to air. Sometimes tap water is quite low in CO2, so shows high pH when you first run it, but picks up CO2 on being exposed to the air, so the pH can drop over several hours to a day or so. This happens fastest if the water is stirred in any way, which mixes the CO2 with the lower levels of water.
2) Tap water and a handful or two of substrate. I would stir this several times a day, no need to add a bubbler. But if you have one, add it. One substrate that I have used is so active I have seen results in just a few hours.
3) Tap water and a bit of wood, similar to what is in the tank, if at all possible. As with suggestion 2), no bubbler needed, but some stirring daily would be good. This one may take some time, if the wood is not very active.
4a, b, c...) Tap water with each fertilizer or other treatment in separate glasses. Stir well, and you ought to be able to see what happens in just an hour or so, but I would run this group out a few days to be sure there are no further changes. This would be especially important if you were adding any pH adjusting materials. (pH Up or Down)