I still say it's the rock, from the photo, the rock looks marbled to me, unless you are testing the entire rock, you can't always tell with a Vinegar test. Back to what Aqua_chem said, Unless the Petco sand is Salt Water sand, there really is nothing else that can drive the PH up so high.
What Chems do you use to prepare your water when doing a change? ie Chlorine Remover, etc?