Black Spot Algae

JoeSmith

Member
Nov 21, 2021
15
Florida
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
Hi,

I noticed some black agae spots in my gunite pool. They are small - about 15 in the entire pool.

I have spot treated them with Trichlor chlorine pucks, and scrubbed them. No success.

Volume 20,000 gallons
SWG salt 3200
pH 7.7
TA 100
CH 280
CYA this is low. I haven’t raised it yet, as the Chlorine is holding well with my SWG at 5 to 6. At one point it was at 10.
My SWG is running at 45%, and I understand that I may be overworking it with low CYA but I am ok with that for now.

I would like to lower the pH to at least 7.6 with acid, but I am concerned about TA going down as well.

I would rather not triple shock my pool.

Please let me know of any ideas on how to deal with the black spots and how to lower the pH while keeping the TA in line.

Thank you!
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2014
33,872
Texas, San Antonio/Marion, South-Central Area
Pool Size
17888
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-45 Plus
Welcome to TFP! :wave: Well, the first thing we need to do is figure out what those spots are from. If not algae as confirmed by vigorous brushing and applying chlorine tabs directly, then perhaps copper? Do you have any dry acid laying around that you can put in a thin sock and rub against a spot as a test? I doubt it's iron, but you could test that by rubbing a Vitamin C tablet on the spot.

 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2014
33,872
Texas, San Antonio/Marion, South-Central Area
Pool Size
17888
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-45 Plus
As for the CYA, even though the angle of the sun may be a bit lower this time of year, I would ensure you have at least 50 to help the SWG work more efficiently. Your pH is fine in the upper 7 range as salt pools tend to settle there anyway, but if you chose to lower it a bit with muriatic acid, your TA has plenty of room to go down. No worries until the TA gets to the 50 range.
 

JoeSmith

Member
Nov 21, 2021
15
Florida
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
We think it’s from a variety of things:

In July we had a new Travertine tile deck installed around our pool. The amount of debris and construction material deposited in the pool was huge. At one point we had different firms of algae growing in the walls - which was shocked away. It took a long while to clean the pool. I’m still vacuuming out the last remnants of sand. But it’s mosty clean now.

Also, in August, some people went swimming in the ocean and then the pool, and I think this is where the algae came from. It started soon after. It was worse then.
 

JoeSmith

Member
Nov 21, 2021
15
Florida
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
As for the CYA, even though the angle of the sun may be a bit lower this time of year, I would ensure you have at least 50 to help the SWG work more efficiently. Your pH is fine in the upper 7 range as salt pools tend to settle there anyway, but if you chose to lower it a bit with muriatic acid, your TA has plenty of room to go down. No worries until the TA gets to the 50 range.
Thank you!
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,916
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Welcome to the forum!
Can you scrape off some of the black and smear it on a paper towel?

Did the construction use any metal tools or other metallic items?
 

JoeSmith

Member
Nov 21, 2021
15
Florida
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
I cannot scrape it off. At this point its embedded.

They sawed a lot of the travertine tiles. Mostly it was just a bunch of debris, dust, cement, grout, and tiles.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,916
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
As Pat suggested put some crushed up Vitamin C tablets in a thin sock. Set it on the stain for several minutes. The saws they used were metal.
 

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JoeSmith

Member
Nov 21, 2021
15
Florida
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
As Pat suggested put some crushed up Vitamin C tablets in a thin sock. Set it on the stain for several minutes. The saws they used were metal.
As Pat suggested put some crushed up Vitamin C tablets in a thin sock. Set it on the stain for several minutes. The saws they used were metal.
Ok, I tried that on a few spots and nothing happened. I’m pretty sure it’s black algae.
 

Katodude

Silver Supporter
Aug 22, 2017
1,748
West Palm Beach/Florida
Pool Size
15000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)

JoeSmith

Member
Nov 21, 2021
15
Florida
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
I tried that - poked the spots with a bamboo stick and small metal tool. They won’t budge.

So far I’ve:
Slammed pool to 30ppm
lowered the pH
tested for phosphates - none
tried vitamin C in sock and directly on spots. - did nothing.
Used Trichlor pucks on spots after brushing with metal brush several times - did nothing except bleach my pool surface.

This is a 20 year old plaster pool.

If it’s not black algae - should I be testing for metals? And if there is some sort of metal problem in the water, how would I go about removing the spots?

Thank you, everyone for your help! I suppose at this point maybe it’s enough to know it’s not black algae.