help identifying a stain

[mappo]

Thanks for the reply and the welcome, aylad. No, I don't know any specifics about the algaecide used.

[PoolDoc]

Black stains that exclude algae sound like copper oxide -- copper algaecides, copper pills, ionizers, Nature2, etc *OR* a pool heater PLUS low pool pH levels can all lead to that problem.

Put a vitamin C tablet on a lightly stained area: if the vitamin C clears it somewhat it's metal in origin and probably copper. Black iron stains are ferrous and soluble, so they are not common in pools.