Calcium (probably in the form of calcium carbonate?) would cause the pH to rise. Other things (substrate, rocks, decor, etc) could be leeching out to cause your pH to rise as well.
Another factor to consider is that while your tap water might be 8.2 straight out of the tap, there is some dissolved CO2 in it, which would slightly lower the pH. If you let the CO2 off gas (just let the water sit for a day), you would probably see the pH go up slightly.
As for decreasing the pH slightly, you can try mixing in a bit more RO/DI water.