Difficult Stain

[learthur]

Difficult stain.....
Last year we had a SWG cell failure and developed an algae bloom...mostly around the top 24 inches of water around the waterline tile. It left a stain.

Currently doing the ascorbic acid for iron. That did remove some iron and my brown plastic pool fittings are all white now.
Stain remains.

So my pH is 6 after ascorbic acid
Alkalinity around 20 after adding muriatic acid
Chlorine 0
Ca 450

Did a Jacks Magic Stain ID kit:
None of the 3 stain ID chemicals removed any stain.
Tried a tricolor puck..negative
liquid bleach ....negative
Ascorbic acid...negative

Muratic Acid does remove it.

So am thinking about draining half the pool to remove the dissolved iron and acid washing the top 24" of plaster.

Suggestions?

Thanks,

Lee

 

[JamesW]

It looks like it might be copper.

It might be black algae.

Can you scrape any of the black off like black mold?

 

[JamesW]

Can you show the plaster when it was new?

 

[learthur]

It looks like it might be copper.

It might be black algae.

Can you scrape any of the black off like black mold?

I scrubbed it with a pool brush and by hand with a chlorine puck. Neither removed it.