Anything Karbonate or bi-carbonate, But you should really read up on buffering capacity and base-creating vs. Acid-creating reactions. Driftwood probably isn't the (direct) reason your PH is shifting like that, though it could be precipitating some solids resulting in a reduced buffering capacity or KH (Karbonate Hardness). I learned a hell of a lot about this from "Ecology of the Planted Aquarium" by diana Walstad. Even if you're not big on plants thats an awesome book
