Black algae?

RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
I have a fiberglass pool and noticed this on our stairs. I scrub pretty hard and it stays. How can you tell if it’s algae vs a stain?

AAA08FA6-3009-4E08-B139-E8699A02B5B1.jpeg
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
A couple things you could do to help us narrow it down:
1 - Post a new full set of water test results. You can show them here and/or in your Poolmath log since it's linked.
2 - Try to rub a Vitamin C tablet on the stained area. It the Vitamin C tabs removes a stain, it's iron. On a well?

Let's start there and see where it takes us. :)
 

RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
A couple things you could do to help us narrow it down:
1 - Post a new full set of water test results. You can show them here and/or in your Poolmath log since it's linked.
2 - Try to rub a Vitamin C tablet on the stained area. It the Vitamin C tabs removes a stain, it's iron. On a well?

Let's start there and see where it takes us. :)
I’ll run a full set of tests tomorrow morning. City water
 

Katodude

Silver Supporter
Aug 22, 2017
1,286
West Palm Beach/Florida
Pool Size
15000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
Take a wooden toothpick or a bamboo bbq skewer and try to pick at it. See if you can pick out a spot or two and smear it on a piece of paper. If it smears green its black algae
 
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RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
I just logged it on poolmath but I have no idea if the CYA reading is correct. I really hate that test. I briefly look at the bottom with my back to the sun and I feel like I could see the dot until 20 really but I don't see how my CYA could possibly be that low. I just added about 1 gallon of instant CYA about a month ago so that should have raised it 16.
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Your Poolmath FC level is nice today, but not sure if it was maintained that way consistently. As noted above, have you tried scraping the spots off and rubbing on a paper towel? If the stain appears embedded, try rubbing with a Vitamin C tablet?
 

RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
My FC was higher for a while until I turned the heater on and raised the temp. I think the FC started to burn off faster so I had to increase my SWG percentage.
Haven't had much time to work on the pool. We had a tornado warning this morning and thunderstorms so I might be able to see over lunch.
 
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RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
80.jpg79.jpg78.jpg

A few more pics. I'm not sure I've been able to scrap anything off to see if it was green. I think I'll have to go buy some ascorbic acid
 

RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
I bought some ascorbic acid and rubbed the spots and nothing happened. I tried again to scrap something, this time with bamboo skewers but nothing came off.
 

YippeeSkippy

Mod Squad
LifeTime Supporter
Jan 17, 2012
16,000
Evans, Georgia
My understandsing is that its very hard to imagine black algae in a fiberglass pool as there is no way for the roots of the algae to "take hold" in the smooth surface. It could be regular algae, stains, dirt, metal stains......

Maddie :flower:
 

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Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Yeah, if the ascorbic acid did nothing, that should rule out iron. There are 3 more possibilities I can think of:
1 - Organic; try rubbing a chlorine tablet on the spot, but don't let it just sit. Keep it moving.
2 - Put a little dry acid in a thin sock and rub it to see if it's copper
3 - Possibly cobalt staining's. We'd have to come back to that one if needed.
 

RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
I tried a chlorine tablet and it didn’t do anything. I added dry acid and rubbed with a sock but it didn’t do much. Then I poured the dry acid on a spot and let it sit for a couple of minutes and that seemed to get one spot ok. It looks like it’s spreading and it seems to stay in the creases of the pool. This is super frustrating, I don’t know what it could be. I haven’t added much water this year and I haven’t even fertilized the lawn yet so if it’s metal I have no idea where it came from.
I had read that black algae was unlikely in a fiberglass pool and the chlorine tablet didn’t do anything
 

YippeeSkippy

Mod Squad
LifeTime Supporter
Jan 17, 2012
16,000
Evans, Georgia
ok, your FC is right on the border.....raise it up and keep it about 9-10 and see if over time and with brushing that will help. Its safe to swim in, so don't worry.
Has anyone added cheap algaecides or used copper in the pool?
Maddie :flower:
 

RHanna

Well-known member
May 12, 2020
90
Gallatin, tn
ok, your FC is right on the border.....raise it up and keep it about 9-10 and see if over time and with brushing that will help. Its safe to swim in, so don't worry.
Has anyone added cheap algaecides or used copper in the pool?
Maddie :flower:
I'm the only one that works on it so no, I've never used algaecide or anything with copper in it as far as I know.

I added 2 gallons of instant CYA yesterday because my CYA was so low and I'm impatient.