At this point I would not add anything, pH of 8.5 is on high end but not bad. What are you using to measure the pH? And do you know your alkalinity? By the way any buffer would not lower it. You would need something like vinegar or other acid. But if you get too much into adding one thing to correct it one way and then add something else to correct it back the other way, you can get into trouble fast.
Those Ph test kits can easily be off several points one way or the other. It is a good idea to leave it alone with Ph reading you are getting. Get some Salifert test kits nest time, they are better tests.
