First time here and going on my third year of owning an in-ground pool. I do pretty well with keeping my water clear after opening, but I've noticed that there is a light brown stain on the walls, steps and floor (but oddly not on the floor of the deep end). I've already done a little reading here, so I placed a chlorine tab on one of the stained steps and left it there for about 10 minutes. After removing it, the stain was completely unchanged, so that rules out an organic stain, right? What should I do next?