The pH of water will rise after it comes out of the tap because CO2 dissolved in the water will come out of solution. The pH will not rise as much as has been reported here unless there is something in the tank forcing the pH to rise. Wood will not raise pH although some will lower it more than others. The first place to look would be the substrate, whether it is sand or gravel. Take a sample of your gravel and put it in a sample of tap water and draw another sample of tap water. Let both stand for a day or so and measure the pH in both containers. If my guess is right, the water with a sample of substrate in it will have a higher pH. If your substrate is driving up the pH the solution is simple but not easy to do. You will need to replace the substrate with something that won't drive the pH up. At a pH of over 8, the ammonia in your water is much more dangerous than it would be at a lower pH so keep a sharp eye out for any ammonia that shows up.