OK,

It is most definitely stain related.

I poured a whole bottle of Vitamin c pills into the bottom of a sock and crushed them up. Got into the shallow end and proceeded to scrub along the wall, the steps and then dropped the sock to the floor, stood on it and dragged it along the bottom and watched as the stain disappeared and the blue liner showed through.

Aside from the method outlined in the previous post, is there a solution that I can buy to add to the water and let it works its magic?