Er, probably not. If anything, real rock might make the water more basic. Leached minerals into the water increases alkalinity which in turn increases PH. But I don't think it happens on a scale that is worth worrying about.
However, putting in a piece of driftwood will lower the PH, as driftwood releases tannins and such into the water. You can minimilize the effect by boiling it or letting it sit in a bucket of water, change the water every few days until the water runs clear