The tannins in driftwood will usually lower the pH in a tank. This is
normal. Do not panic.

Squid wrote:

This morning I added two pieces of driftwood to my 20 Gallon tank that
already had another piece of driftwood in it. Before doing this my PH
was about 7.4 - after adding the additional driftwood almost instantly
my PH dropped to 6.2 and now seems to be hanging around 6.5 - is this
normal for driftwood to make such a big difference like this? Not that
i'm complaining, I just wasn't expecting much of a resutl