Black spot algae?

Belldiver

Well-known member
Sep 7, 2007
344
Lafayette, Louisiana
Hi folks!

I keep getting some little 1/4" round black algae spots in my pool. My numbers seem good and I'm not consuming chlorine overnight, but these little buggers keep showing up. Fairly sure they're some type of algae, they will scrub off, but takes a little effort with a stiff brush. If you look closely at them they do have a green tinge, but from a few feet away they look black. They tend to stick in the usual algae hangouts. Ridges in the plaster, corners, etc... This isn't a major outbreak, but I can't figure out why they keep coming back.

Any thoughts on what type of algae this is? Not mustard right? Why is 6.5 FC not killing this stuff and letting it regrow?

pH: 7.5
FC: 6.5
CC: ~0
TA: 70
CH: 330
CYA: 35 (will bring up to ~50-60)
Salt: 3300
Borates: 15
Temp: 87

Thanks,
 

frustratedpoolmom

TFP Expert
LifeTime Supporter
In The Industry
May 20, 2007
12,177
SWSuburban Chicago, IL
How often do you run the filter, any daytime run hours?

Black Algae has a tough outer shell that is resistant to "normal" chlorine levels... the key to getting rid of it is brushing to knock of the outer shell so the chlorine can attack the insides.

How often do you brush the pool?
 

Belldiver

Well-known member
Sep 7, 2007
344
Lafayette, Louisiana
Thanks for the reply,

The pool filter runs 24/7. Lots of activity in the pool. I brush and run the Polaris 280 every few days. These spots do seem to have somewhat of a shell and once you break that off it goes away. I just can't figure out why they keep popping up randomly with a stout FC level.

Jim
 

duraleigh

Admin
Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
31,540
Sebring, Florida
It certainly has all the characteristics of black algae. Black algae is probably the most mis-diagnosed type on the forum and most folks don't have it when they think they do for whatever reason.....your description sounds like you have it.

Why is 6.5 FC not killing this stuff and letting it regrow?
YOU need more chlorine....it really that simple. I would shock the pool and hold tyhe pool at shock until I thought the algae was gone. I would also suggest engaging it with a stainless steel brush while you are at shock.....more than once.

Then, when it appears all gone, you will need to hold your FC level up higher than normal for virtually the remaining life of the pool. No big deal....just keep your FC higher than normal.

So when you elevate your FC to 50-60 I would suggest you maintain FC levels of roughly around 10 at all times. Certainly never below about 7.
 

Belldiver

Well-known member
Sep 7, 2007
344
Lafayette, Louisiana
The pool is about 2 1/2 years old and I've never had much of a problem with algae until this last little outbreak when I got back from a couple months away from home. The pool got occasional maintenance from some friends, but with the heat of summer ended up catching an algae bloom the week before we got home.

Is black algae some type of HIV for pools? Can't I just shock this away good and hard? I always thought mustard algae was the bad one?

Thanks,
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
You have to kill black algae at its roots and that means scraping off the chlorine-resistant heads to expose the lower portions of the algae to high shock levels of chlorine. It is possible to kill it off this way, but like yellow/mustard algae you really have to work at it. It's not just a simple shock and you're done as it is with most green algae. It takes more brushing and sustained shock levels. Some people rub Trichlor pucks in the area of algae as well.

Fortunately, with your pool not being very old the plaster (diamond brite) should be in good shape and not terribly pitted. It would be harder to get rid of black algae in an older pitted pool since the algae could really dig in deep and therefore be harder to kill for good.
 

Belldiver

Well-known member
Sep 7, 2007
344
Lafayette, Louisiana
Thanks for the input,

I may run out tomorrow and get a new stiff brush. My older blended brush just barely gets these things and the robot cruises right over them. And I'll shock it up above 20 this time, instead of the usual 15 or so.

Jim
 

michmike

LifeTime Supporter
Apr 11, 2011
105
Houston, TX, USA
hi there. did you have any luck removing the black algae spots? if yes, how did you do it.
I got some of these myself and the pool store recommended two things:
-use a stainless steel brush to kill their heads and expose them to chlorine
-and use some black algae cleaner solution.

I will start with following the advice from this thread. use a strong brush to expose the heads and elevate to shock level for a few days and see what happens.

cheers.
 

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