What is this black stuff?

Frank in FL

Well-known member
Jul 3, 2019
423
Florida
Pool Size
16500
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Jandy Truclear / Ei
A friend of mine has a fiberglass pool. Over the last six months these black spots have appeared throughout the pool.
The ones right below the waterline I tried to scrub off with my fingernail and it was not budging. Almost feels like you would need to take some sandpaper to get it out of the fiberglass. It looks like something burned under water causing these little black spots but he assured me that is not the case.
 

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I will bet it is Cobalt. Do some research. There are some products that state they will remove it. It is a reaction more than a stain.
 
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