How low can driftwood bring my ph?

Driftwood can bring PH down, but it depends on the water hardness (GH) how much it would bring it down in the first place. and 7.2 is great for most freshwater fish. On planted tanks, the PH equalizes at about 7.3 ~ 7.6. They are some that inject CO2 in , but they pay the price as they replace thier plants more frequent, and some actually become sterile, or the leaves take a different shape. If you use soil, you wont have problems with ph bouncing as the hardness goes through the roof and ph is usually 7.3~7.6; the only thing you have to worry about is nitrate and Ammonia Levels for fish . Oh, and dont be worried when the water yellows, thats because it is releasing tannons from the driftwood.