Different rocks leach different elements into the water. Lime stone leaches calcium which raise the pH. Low pH usually comes from having drift wood. To test rocks to see if they're calcareous take white vinegar, spill some on the rock, and see if the rock bubbles (if it does, it means its made with calcium carbonate and will raise pH). I can't remember off hand what rocks lower pH.
We all know water is one of the main points in our hobby but we sometimes forget how much water can vary from place to place. What works even for the North end of Ohio doesn't work the same for the South end. In many places, most rocks are very safe. If we get into areas that have acidic water, rocks may change things but if it is rock from nature put into the water found in nature in that area, there may not be any change noticed. The rocks and water have spend at least a few thousand years together already.