Using pH Up is a big mistake as there is nothing to keep the pH buffered. All it will do is make your pH swing wildly and probably kill the fish. Better to have a stable low pH than a wildly variable one.
I have huge bits of driftwood in my tank and it has never altered the pH, depends on the type of wood I guess.
If it is worrying you, put some shell or bird grit (the stuff you buy for chooks) in the filter. That should keep the pH up a bit and stable.