dark ring around the water line

[Nova13]

I have a fiberglas pool and I'm seeing a dark color waterline ring.

I'm scrubbing but it still remains,

the source water is filtered thru carbon at 5 micron, so it not the source.

What's the cause?

What to use to clean it off?

 

[Texas Splash]

Normally this sounds like a scum ring. I get the same thing, but then I can scrub it off and it smears with my finger. In my example it's causes by oils on the surface of the water that accumulate against one corner or side of the pool - most often when the wind pushes in that direction. But you are scrubbing and it's not coming-off? Have you tried a green scrub pad, or perhaps a Magic Eraser? Maybe on the scrub pad also use a mixture/ratio of 4/1 bleach and water?

If these still don't work, please post a full set of test results and perhaps we will see some other indicators to your problem.