Black Algae - can/should i use polyquat when shocking?

busbeepbeep

Member
Jul 5, 2007
24
New Orleans, LA
I just spent a couple of hours swimming in the pool scrubbing black algae off the walls using a trichlor puck and a SS brush. I managed to get a lot of it to break off wherever it was protruding from the wall, but there is a lot remaining. Lots of pits and grooves in the plaster (~ 25 yrs old).

I'm pretty positive it is black algae. The spots are round, and they smeared dark green when rubbing off.

What should be my shock level to try to get rid of this black algae from ****? I have a bottle of Polyquat 60. Will that help? I read it can help break down the chlorine resistant parts of the algae.

Any suggestions?

Test Results:

FC 5
CC 0
TA 80
PH 7.4
CH 500
CYA 25
Borates : hopefully 50ish, added yesterday, didn't try test strip yet.

Besides the black spots, the water looks better than ever.
 

JasonLion

TFP Expert
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May 7, 2007
37,880
Silver Spring, MD
Regular shock level is fine. With CYA around 25, shock level is 12.

Adding polyquat won't help very much, and probably isn't worth the expense. Using borates will help just a little. Frequent brushing while at shock level helps a lot, as does rubbing with trichlor tablets.
 

frustratedpoolmom

TFP Expert
LifeTime Supporter
In The Industry
May 20, 2007
12,177
SWSuburban Chicago, IL
With black algae, all you really need is chlorine and elbow grease. The pq 60 won't really help you here.
The trick with the BA is to brush it daily to kill the roots - the outer shell is resistant to chlorine so by brushing it you allow the chlorine to attack the insides.

Normal shock level, 12 is fine. :)
:goodjob: Jason and I are typing at the same time :mrgreen:
 

X-PertPool

TFP Expert
In The Industry
Jun 12, 2009
1,385
Exeter, PA
It smeared green? usually when I brush off black algae the round disc of black algae gets ripped off leaving a small ring where the algae was at. I don't usually see any green though.
 

busbeepbeep

Member
Jul 5, 2007
24
New Orleans, LA
Mostly it just flaked off, some times it looked a bit dark green.

Thanks for the advice. Thats what I did last night, I left the PQ out and shocked to 12-13. Added bleach twice to get back to 12. Not sure what the overnight loss was though, didn't get a chance to check until 1pm. With black algae taking a long time to get rid of, will I ever get an overnight loss of 1 or will I have to shock for a long duration?
 

frustratedpoolmom

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May 20, 2007
12,177
SWSuburban Chicago, IL
It's more about the brushing than the chlorine level - you may not even get an overnight loss greater than 1 (more than likely you will tho). I'd start out by shocking for a day or two and then just keep your FC higher than normal, make your "target" your "min" - does that make sense? and then just brush brush brush. That way you can keep using the pool...brush brush brush! :)
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
Black algae smeared on a white surface will be dark green. It's algae and has green chlorophyll though isn't visible until thinned out by smearing.
 

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