It would be good to talk to ...

It would be good to talk to your county extension agent after you get a water quality analysis done (many state departments of health will provide an analysis at low cost). The marble may work, perhaps you can add it and experiment with marble and some water, running pH each day to see how quickly it will dissolve. There are other forms of calcium carbonate that can be added like powdered lime and you would need information on how much water approximately you will be treating. Many times pinhole leaks and the color you describe are the result of copper pipes corroding. The pH for copper should be above 7.4. The water quality analysis and county extension agent may be able to give you some tips for well water pH increase. Hope this helps.