I had gray blacking stains throughout the pool. I had water tested several times and no testing turned any reason up for the stains. The first year the pool was built i had a copper ionizer system. It was a nightmare. After having the pool acid washed , i switched to a salt system. All the walls of the pool were covered in a grey/black streaking that would not scrub off or go away with the lemon test and no chemicals ever came out in testing. Now round two of acid washing. This guy seems very knowledgeable but i am concerned. After acid washing the walls of the pool are definitely shaded where the stains were. I am not sure if they will be visible on the photos i am attaching. Is there anything else before i refill the pool to improve the appearance of the pool? Is there any chemical to add weekly or monthly to prevent an unknown stain from re occurring?