One of two things will remove all stains from pools. If it is a metal based stain Ascorbic Acid commonly sold as Stain Free will remove metal from pool surfaces and put the metal back into solution (into the water) for removal via the filter – usually with a filter aid such as FerriTabs for sand filters or SeaKlear.

If the stain is organic, from leaves/algae etc. it will be removable with chlorine, and in extreme cases muriatic acid.

If neither of these work, and it’s a liner pool it could be a form of algae that grows up through liners and in our experience is impossible to remove. Sometimes if the fungus/algae/liner mold has grown for some time, even replacing the liner will have it growing through the new one.