What is this darkening in the pool?

[Thethreetheory77]

Can someone give me an idea of what this darkening is in the pool? They are dark streaks and big blotches on the bottom of the pool. Have no idea what they could be except maybe due to my calcium hardness at 200? I know I need to bring that up but I have no idea what is causing this.

 

[ajw22]

How old is the plaster?

Could be mottling of the plaster.

Or could be metal staining.

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