Staining of pool surfaces might be caused by:
a: Mineral salts in the pool water.
b: Mineral salts in tap and borehole water.
Algaecides lose their stability and become less soluble than normal and in doing so can cause staining in varying degrees. Read more about this at the AlgiHalt page.
Most surface stains are caused by excessive mineral deposits that can be removed with IronCrash.
Remember that the older the stains the deeper it penetrates into the pool surface and the more difficult it will be to remove.
For instructions and more information view the Guide.