Pebble Tec Pool Help

[debrr]

I am a new pool owner and sure don't know what's going on. I hope I am posting in right place.
I have a salt water pool with only pebble tec on the interior (no tile). The pool builder gave me a nylon brush to scrub the walls of the pool but it hasn't cut it. I went to a pool store and they said to use this combination brush (stainless and nylon bristles, I think) Bottom line is that I tried to use it around the water line and while it was an improvement over the nylon brush it still isn't getting all the staining off at the water line. I have only had my pool filled for about 3 or 4 months.
Can anyone tell me how to get the discoloration at the water line off?
Thanks!

[Pool Staining]

Is the staining rough or is it more like a scummy dark line?

If rough, then you probably have a scale problem, which is often the result of raised total hardness from the increased salt content in the water.

If it is a dark "tarry" layer, you can find many products to remove the built up oils which cause the line.