Plaster pool stains

Got a new house last year and this year, after removing the cover yesterday, the walls above the water line are stained (brown). Some websites indicate that an acid wash is required. Is that the case? Or there is an alternative such as brushing the walls with liquid chlorine?
Thank you

It depends on the stain.

Muriatic acid solution will remove iron stains. So will Iron Out. Test before you proceed! Acid washing often damages pools.

Bleach (liquid chlorine) will remove algae and some organic stains.

Other things, remove other stains.

Ben