Ok, first off, adjusting tap water is tricky at best, and deadly at worst. Unless you have really soft water you will probably have pH swings, something that fish really don't like. Consider yourself warned.
Baking soda will raise pH, driftwood, bogwood, or peat will lower it. Start off small, give it a few days and test twice daily. You may want to experiment in a small tank without fish, or a large bucket.