What Happens to Cl Levels When Algaecide is Added to Your Pool Water?


Sep 30, 2019
Sarasota, FL
I figured that adding a maintenance dose of algaecide every week would allow me to reduce my SWG output of chlorine making it (the generator cell) last longer. I normally run a steady free Cl level of 4 to 6. I added some algaecide. Days later some visible algae started in places. I noticed that my free chlorine levels dropped by 4 or 5 PPM a day or two after adding the algaecide. Sometimes it was as low as 0-1.

So I did a little experiment over the last few days regarding algaecide and the effects on chlorine levels.

I used a 5 gal. plastic bucket and filled it with pool water. It was kept under cover in the shade and the bucket opening was covered with the fitting plastic top. A separate 1 qt. bottle was also collected as a control.

Day temp. Max: 84F
Night temp. Min: 80F
Dew point: 75 F
Avg. Pressure: 29.9 in.
Winds: 16 MPH, Gusts to 26, then calm the second 24 hours
Sky: 100 % clouds

Pool water, no Algaecide. None was used for several months now. After baseline tests the maintenance dose of a “polyquat 60” algaecide was added and stirred into the 5 gallon bucket. This contained no copper. The control bottle was untouched.

Baseline test, no algaecide, hour zero:

TA: 98
CH: 268
CYA: 40
Cl/free: 7.8
Cl/combined: Forgot to measure but usually around 0.0 to 0.2
pH: 7.24
NaCl: 3700
TDS: 4080

At 6 hrs. time after the maintenance dose of algaecide addition:
TA: 106
CH: 273
CYA: 40
Cl/free: 7.1
Cl/combined: 0.33
pH: 7.26
NaCl: 3710
TDS: 4080

At 12 hrs. time:
TA: 103
CH: 257
CYA: 38
Cl/free: 6.4
Cl/combined: 0.40
pH: 7.35
NaCl: 3720
TDS: 4110

At 24hrs. time:
TA: 110
CH: 274
CYA: 41
Cl/free: 5.5
Cl/combined: 0.36
pH: 7.27
NaCl: 3720
TDS: 4090

After the 24 hour sample was taken and measured I added in the “visable algae in the pool’ dose of Algaecide. The bucket was stirred. Another 24hrs. later I took more readings:

TA: 111
CH: 260
CYA: 40
Cl/free: 0.28
Cl/combined: 1.83
pH: 7.39
NaCl: 3730
TDS: 4080

The CONTROL free chlorine level dropped 0.5 PPM during the 48 hour test (no algaecide was added to this original separate sample).

There are suggestions that SWG cells can effect the CYA, polyquat and Cl interactions or such but that cannot have any effect in my 5 gal bucket. I read that “high” chlorine levels can inactivate some algaecides. If algaecide can decrease chlorine levels and chlorine can inactivate algaecide this may explain my findings - some algae streaks in the pool. Several people have said that algaecides should not be used and are not necessary at all. Chlorine does it all if kept at the appropriate levels. ‘True for algae blooms - use only chlorine at the appropriate levels and time - SLAM.

If this is all true then why do people recommend algaecide use and why do they sell algaecide?

FYI - Samples were tested with a photometer. Chlorine calibration at 1.8 measured 1.8 on the meter so I am accurate to 0.1 exactly. I also use a Tylor Kit K-1741C with fresh chemicals to secondarily verify findings, at least to make sure they are in the ballpark. This kit uses slide comparators for accuracy beyond that capable with normal color matching techniques. Biochemists as myself do not always trust single metering devices on critical applications. I trust the accuracy, resolution and stability of my photometer.

There are a great many good articles about pools. Obviously a lot of people like: Home - Trouble Free Pool.
I like this summary in general as a PDF:

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Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
What specific algaecide product are you talking about and what chemicals does it contain?

We recommended never adding products to your pool that you do not know the chemicals in it and what they will do to your water chemistry.

Algaecides are money makers for pool stores and are sold as quick fix potions for water chemistry problems.


Bronze Supporter
Aug 15, 2017
I figured that adding a maintenance dose of algaecide every week would allow me to reduce my SWG output of chlorine making it (the generator cell) last longer.

I don't think the use of algaecide allows you to lower your target chlorine level, otherwise it would be an alternative to chlorine and it's not. It does not take the place of chlorine. That's likely the reason for your algae growth.

Another thing to consider is that the photometer may be accurate at 1.8mg/L, but may not be accurate at a much higher level (6-8mg/L). In order to assure your photometer is accurate across the range, you'd need to do a cal at a low level and then one at the high level, to in essence bracket your expected result. Results from titration chemistries generally allow for accurate analysis of a much broader range.
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Sep 30, 2019
Sarasota, FL
So it seems that algaecide should only be used for closing a pool. Polyquat 60 will be degraded by chlorine, and by this experiment chlorine will be degraded by Polyquat 60. After reading Algaecide - Further Reading it is clear why Polyquat 60 is useful for closing the pool. So the next question is, living in tropical Florida, what do I do with my bottles of algaecide?



Mod Squad
TFP Expert
LifeTime Supporter
May 11, 2014
Franklin, NC
The problem with algecide use in my opinion....

I see algae as the canary in the coal mine. If you add something to kill algae (copper, quat or other algecides) these "somethings" may not or can't kill the bad nasties that cause illness or death.

Chlorine does it all.
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