You can also easily make your own pH cal solutions if you dont want to pay for them.
If you find its really that high (doubtful, but possible), spend $5 and toss 1 teaspoon of yeast, 1 cup of sugar, into a 2L bottle and fill it up about 2/3rds of the way with hot tap water. Drill a little hole in the cap or top of the bottle and glue in a chunk of small air tubeing line. Stick this line somewhere down low in the tank, or set a small bowl in the tank and stick the end of the line under that. The bottle will supply a steady constant (as long as the reaction in the bottle is kept anerobic) of CO2 into the water for a few weeks. The ratios of yeast to sugar I gave you is more of a high output rather than long duration combo, you can easily tailor the solution to produce CO2 at a constant rate for about 2months if desired.
Anyways, as you well know, the CO2 collected under the bowl will disolve into the water makeing carbonic acid which will very safely lower the pH. Likely will do someother bennifical things for the tank as well if you have macro algaes.
But, most likely, its a probe related issue.