Black lumps/spots on steps - turn green if you rub them

rpoldervaart

Member
Nov 23, 2007
17
I have a few black spots showing up on my top step. They are a raised round spot, looks black, and if rubbed, I can see it smear green. Is this black algae? It doesn't seem to look like pictures I've seen others post. Just a few raised 1/4 inch diameter spots here and there. I don't think I've ever had anything but FC = TC and no CC, but yesterday noticed a slight bit of red tint when testing for CC. My FC got low and I didn't notice right away.

FC 1.5 - usually kept around 4-5
CC barely .5
pH 7.5
Alk 110
CYA 60
Salt 3200
I don't remember CA right off hand, but think it was 260 or so.

I was going to raise the CYA back up to 80 (we've had a lot of rain and a lot of water has gone out the overflow), and then set the pool up with borates. But figure I need to tackle these black spots first. Shocked last night with 6% bleach to about 17 FC. I didn't think about leaving it high for a couple days, so it's probably dropped some today in the sun while I'm at work. Will scrub the spots with a wooden brush tonight. I guess I need to keep the FC high for a couple days, brush the spots, and see if they go away.

How high do you think I should bring the FC?

Thanks,
Robert
21000 gal, in ground, salt Aqualogic PS8
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
That does sound like black algae, more or less round black spots that are slightly slimy and when a bit is scraped off with a finger nail and smudged onto a white paper/card shows green.

Try semi-gently (do more than pat it, but don't grind it down) rubbing one of the spots with a trichlor tablet and see if it will go away that way.
 

Isaac-1

TFP Expert
LifeTime Supporter
May 10, 2010
6,711
SW Louisiana
I am wondering if your top step may be part way out of the water, allowing it to trap water then let the sun drive off some/all of the chlorine thereby giving some type of algae a place to grow. I have had a similar sort of problem due to condensation on my brass shut off valve for the pool fill.

Ike
 

rpoldervaart

Member
Nov 23, 2007
17
Sorry for the delay and thanks for the responses.

The step is about 4-6 inches below the surface. The spots tend to be toward the wall, so I'm guessing there is very little circulation right there.

I bought a trichloride puck and rubbed it on the spots. Kind of hard to say whether I rubbed too hard or not, but I tried to rub without grinding the spot away. The spots turned brown in a few minutes. I'm going to try to see if they will brush away now.

I've retested the water and have the following:
FC 4.5
CC 0
pH 7.5
Alk 80 (lowered it by acid / aeration - going to go down to 70 and then adding borates)
cya 80 - added stabilizer
salt - 3400 - added some salt

Seeing that I don't have any CC, I guess I'll just spot treat the spots with a brush and the chlorine puck, and see if I can't get it cleaned up that way.

Thanks,

Robert
 

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