Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Hard water is usually GH, Calcium and Magnesium. These do not do much about the pH.
KH, carbonate hardness, is the hardness that buffers the pH. Low KH generally means low pH, and it is easy to change the pH. When there is little to no KH, something else controls the pH. Often this is organic matter and decomposing things in the tank. These processes usually create more acidic water.
If your tap water usually has GH and KH roughly equal, but the tank has very low KH, then I would suspect the substrate. Several of them will remove the KH from the water, allowing the pH to be controlled by something else. The high tannic acid materials you added will sure drop the pH! I do not know if that effect lingers, though, once you have done a massive water change and removed the leaves etc. If the substrate removes KH, then it can keep on doing that for quite a while. If you have a little left over try a test in a bucket with a handful or two of substrate and a few cups of water. Check GH, KH, pH when you start, and a day or so later, then several days later.