The ph in my previous 55 gal was 8.5 the other day. The ph in my new 120 gal is 8.5. My test kit only measures up to 8.5 so I'm thinking it could actually be higher than that. Fish seem fine. It's all they've ever known. Should I be concerned and how should I go about lowering it?

Have you tested your tap water? There are various buffers such as wood that will bring the pH down. However, if tap water pH is high, it will obviously change every time you carry out a water change.

Some ph testers do the full range and some only do parts of the range. If your ph tester only goes up to ph 8.5 and your ph is higher than that then you result will be 8.5. To find out for sure you can buy a tester than goes higher than 8.5. However if your fish have been ok so far and they have grown up in the tank and have reached full size without problems then you will be ok.

You can buy some ph down liquid to lower the ph but you must keep the ph the same all the time. Varying ph can be very basd for fish

When bringing the ph down DO IT VERY SLOWLY. Say 0.2 every week

What fish are you keeping. We all assume oscars but more and more members are keeping other fish along with oscars. if all seams well I wouldn't change it. It goes to show other members who are worried with high ph that they have little to worry about if oscars are doing well in 8.5. sudden changes are what they don't like, i also think it makes a difference with Nitrates too. lowering them slowly is better than a drastic multi day change of water. I agree with delboy if you do decide to lower it do it slowly and try to keep it stable at each stage before you drop anymore.