daisy11 is exactly right, you can often lift stains by lowering the ph to 7 and putting in the sequeterer. It does matter how long the stains are there, and a vinyl liner is easier to lift the stains from. Also if you have a fiberglass pool, they tend to stain much easier, and more often. Another thing that works with a fiberglass pool if you only have a few noticeable stains is to get a very fine sandpaper and sand the stain. If the whole pool is stained, you will need to do a treatment, either with the sequestering agent (you should try this first, because you need it in the water with the ascorbic acid), and if that doesn't work, then the ascorbic acid. The reason that metals fall out of solution is with a combination of high ph along with high chlorine. There are many ways to get metals into the water - fill source, and plumbing. and wind blown.