Small Algae Problem w/SWG

slange316

Well-known member
Jun 2, 2010
50
This is my first year with a SWG. It has been perfect. I live in the west burbs of Chicago. When the weather got cooler last weekend and less people in the pool, I thought it would be a good idea to turn down (from what I had set for the summer) the SWG from 20% to 10% and cut the pump time from 12hrs to 7hrs. FC had been at 5-7, CHY 70-80, PH 7.4 all summer.


Well, bad idea. After about 2 days, I noticed some translucent green algae on the bottom. I have 27' round AGP. Here are the numbers at the first sign of algae;

FC 3.5
CYH 70
CC .5
PH 7.4
TA 70
CH 250
Salt 3600-3700

Per the pool calc, I added 2.5 gallons of 12% liquid chlorine, turned off the SWG, let the pump run. This brought FC to 28. After 48 hours FC is 22, CC .5. I have turned the SWG back on to help oxidize the .5 CC.

Water is crystal clear, no signs of algae on the bottom. I am a little concerned about the CC but the SWG should take care of it. But how will the FC get down to 5-7 if I am adding FC via SWG? 2nd, how could I have algae growing when the FC was at 3.5? Was this the effect of the SWG just not able to keep up at 7hrs/10%? There was still 3.5 FC?
 

Bama Rambler

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2009
23,036
SouthWest Alabama
Did you pass the OCLT (overnight chlorine loss test)? You're not done shocking until you do.

Algae can get started in areas of low FC caused by poor circulation. That's most likely what happened in your case.
 

slange316

Well-known member
Jun 2, 2010
50
It has been rather windy hear so I am getting a lot of Crud blowing into the pool. I feel the OCLT would not be totally reliable so I have been testing CC. After the first 24hrs, CC=0. It was up to .5 or less after 48hrs.
 

Bama Rambler

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2009
23,036
SouthWest Alabama
The OCLT is very important because you can fail it and have zero CC's or you can pass it and have .5ppm. I'd run it anyway even if stuff was blowing in the pool.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
slange316 said:
how could I have algae growing when the FC was at 3.5? Was this the effect of the SWG just not able to keep up at 7hrs/10%? There was still 3.5 FC?
3.5 ppm FC with 70 ppm CYA is right near the minimum for an SWG. So yes, if the circulation isn't good in all areas of the pool, algae can start to take hold and grow and use up chlorine locally. Also, when you cut down the pump run time, there was a longer period of time when the FC level could drop so the level could have been lower than 3.5 before the SWG turned on again.

For your pool, perhaps you don't need 20% SWG with 12 hours of pump time, but cutting SWG output in half was probably too much and maybe cutting down the pump run time was too much as well. You can try splitting your pump run time to be something like 6 hours day and 2 hours night to not have such a long gap of still water, though it's usually better to run it longer at lower speed if you can. You can also try a 15% SWG setting to see if that can maintain at least 4 ppm FC to be on the safe side.

Every pool is a bit different and you need to figure out what is reasonable for your specific situation.
 

slange316

Well-known member
Jun 2, 2010
50
It passed OCLT. At this rate though, it will be several days before FC drops back to a swimmable level. Thanks for your help.

slange316
 

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