I know there are some types of rocks that can affect tank water. At the moment I can't remember what they exactly do to the pH, but I'm sure it's possible it could lower it. Whenever I've added rocks from directly outside, I always soak them in a buckey of water with an airstone for a week, then test my water.
I can atest to that buffer crap messing up water pH. When I first started fish keeping, I took my water in to get tested by a chain store, and the pH was about 8 or 9 (which was what it was out of the tap), and they said I had to get it down to 7 with the buffer. My pH was constantly up and down...what a headache. As soon as I stopped using the pH buffer, my water settled down.