It might take a few water changes to get the carbonates down to a lower range. As long as there are noticeable carbonates in your water (dissolved from the coral), they are going to buffer the water, keeping the pH up a bit higher.
Maybe post some pics of your rocks? Just because a rock is sold in a pet store doesn't mean it won't alter pH. A lot of the rocks used in the ADA- style scapes are just really weathered limestone, and those can affect hardness and pH. Also, almost anything used for marine setups is going to be carbonate as well.