Plants sound fine, and a few days to make sure they are growing and not dying is good, so this fits in with the water change/stability issue. Do you have/use any fertilizer, and if yes, which?
Organics like wood can affect parameters, but there are some other factors that have more of an impact on parameters. What is the GH and KH (Alkalinity) and pH of your source (tap) water on its own? GH and KH will determine what the pH may do, since they act as buffers. If they are high, the organics w2ill have no effect on the pH, but if the GH and KH are quite low, then the organics can be more of an impact. CO2 enter the picture as well, and any other things like calcareous items, etc.