I have had a 12 yr old 15x30 above ground pool suddenly turned black after shocking it with granular chlorine what should you do?

There is a component of some brands of chlorine that can cause a black coating to come over the bottom of fiberglass pools. To remove it I recommend several bottles (3-5) of Regal Stain Remover (or any comprable brand). Let the stain remover set for an hour before scrubbing the black off. If this doesn't work, it's always a good idea to contact your local pool professional. In Dallas, TX Pure Pools is your best bet. Good luck.