We have a small pool route - about 114 pools. Two of our pools that we acquired from someone else have a dark black ring which is in the steps along the tile line and along the top of the walls - again where the tile meets the plaster. We were told that it is called "hydration". Would anyone please explain what this is. Will any kind of "metal out" take care of it? I tried a vitamin C tablet today but it didn't work.