Hoping someone can help. Fiberglass pool with SWG and waterline tile. Pool is about 3 years old. The side that is full shade has black stains starting to appear in the grout. I have tried chlorine in a spray bottle with a brush. No luck. I have tried soft scrub and a brush. No luck. Lastly I tried a stain remover granular product in a sock that you scrub against the tiles. No luck.
The black strain will not rub off with my fingernail and almost seems to be "in" the grout.
It exists below the waterline, at the waterline and above the water line.
Any ideas on how to remove? Is it unsafe in any way?