What do you make of this stain?

[Oknoproblem]

Hi, need some assistance with this stain. I live in Fl and have a 22,000 gal. pebble-tech sand finish pool. Previous owner of the house had installed gutters but didn't drain the water away from the pool so when there was as storm the pool filed up and overflowed the paver coping. As a result I have this nice stain around half my pool. I have tried chlorine and acid, (scrubbing with a stiff nylon brush) no change. Any suggestions wold be greatly appreciated.
Thanks in advance.

 

[duraleigh]

Hard to identify. An easy possible fix is to elevate the pool level until the stain is submersed and then run your chlorine about halfway in between normal and shock level (per the FC/CYA chart) for several days. If it's organic, that should remove or lighten it noticable. Brushing during that time is always beneficial.

 

[Oknoproblem]

Thanks for the suggestion but that's really not an option as I would have to overflow the pool to get to that level, and I did try to treat it with Chlorine, but I admit it was a limited duration, maybe 5 minutes during brushing before washing it off into pool.