Is this a sudden increase? or have you only just tested? How long has your tank been matured? How long ago did you put your rock in (and what rock type is it)? What are your other water parameters, and have they changed recently too? How do the readings compare with your tap water?
I would strongly recommend that you do not chemically try to bring down your PH (and I am sure you mean GH/KH when you refer to alkalinity? High PH *is* alkaline...)
Using chemicals to bring down your PH will only stress the fish, since they are more sensitive to PH swings, than they are to living in water with a PH reading not usually recommended for that type of fish. Also, the likelihood is that the levels will climb again anyway - especially if your tap water reads high, each water change will make sure of this (the buffering levels will also impact on the effectiveness of any product such as PH Down). High PH is associated with hard water and, in turn, high buffer (GH/KH) Most fish can live in water that differs from their "comfortable PH levels" if they have been introduced slowly and carefully. Also, the chances are your LFS will have the same high levels in their water too.
If you are sincerely worried, and the change is recent, then take out the rock, spend a few days performing partial water changes, then invest in a new test kit. I would not continue to use a test kit just because I had some left, if I was not confident that the readings were accurate.
Good luck!
