Is this Algae or something else?

[KSpoolguy]

This black spots are showing up above the water line in random spots. They start off almost light black or gray and are easy to remove at that point. Once they turn black, I am finding them almost impossible to remove. The spots are slightly raised and become more prominently raised when black. Any ideas on what it is and how to remove it?

I initially thought some sort of algae. Then thought maybe metal stains. We did use about 1/3 well water 5 years ago when we built the pool for the initial fill and used metal sequestrant. The next summer we had to drain the pool half way down for the builder to fix a liner issue and only used city water to refill. Year 3 we had a small hole and kept filling it back up with city water. And in between these times we only used city water to refill after evaporation, splash out, and after backwashing. So I would guess there is very little well water (which does have a lot of iron in it) left in this pool. No stains below water line. Our sprinklers (on well water) do occasionally hit the pool, but just minimally.

Test Results this morning:

FC: 7.0
CC: 0.5
PH: 7.5
TA: 120
CH: 225
CYA: 40
Borates: 30
Temp: 88

Thanks in advance!!

 

[Mendy48]

How does it feel when you touch it?

You should do an Overnight FC Loss Test (OCLT) to see if there's any organics in your pool.

 

[mknauss]

Does bleach remove/lighten them?

 

[KSpoolguy]

Before it turns solid black, it's a bit smooth and can almost be removed by rubbing it with your finger (the residue is similar to removing sticky residue from a sticker). Once it turns black, it's hard and rough.

I tested my FC and CC last night before I went to bed and my FC was 7.5 and CC was 0.0. So my FC only went down 0.5 overnight.

I am getting ready to apply a little bit of bleach to the area to see what happens and will report back.

I really appreciate the help!!

 

[KSpoolguy]

Rubbed some bleach and also a puck on the black spots with no color difference.

 

[mknauss]

So, they are not organic.

Next step is metals. Any copper or silver or zinc containing products used in this pool? Algaecides, etc? PoolRx?

Dry acid in a sock will test if copper is present. Use all normal precautions when handling it.