Free chlorine disappearing more quickly than before... thoughts??


Silver Supporter
Jun 21, 2018
Woodbury, MN
Hi all,

Having been having some issues with chlorine levels lately so thought I would reach out for thoughts/insights.
Up until 2 weeks ago, we were enjoying a magnificent summer with the pool. the water chemistry was in perfect balance, I had the SWG dialed (have been running it at about 15% pump run time all summer) in and was needing to add 1-2 jugs of 8.25% bleach every week if even that often. water was crystal clear. I was losing about 0.2-0.7 ppm FC per day, depending on usage; never more than 1ppm per day.

Then about a week ago, I noticed a bunch of air in the pump basket. I did the standard trouble shooting, looking for suction side leaks, of which I found none. I figured the water was circulating very poorly at this time. I even lost prime in the pump basket one time. Then I asked around and swapped out the filter cartridges thinking they were the reason for poor pump performance, and the pump is now back to normal with water filled all the way to the plastic lid on the top of the basket. During this time there was a period where my FC went from 8ppm to 0 in a few days, which has NEVER happened before. combined chlorine has ALWAYS been 0.5 or less. Also noticed the deep end was looking a bit cloudy. So I quickly added a bunch of bleach and some CYA (because my CYA had also gone from ~70 to 40-50) to bring FC to SLAM level. The pool cleared up quickly, but now I notice that the FC is dissipating much faster than it used to. Here are some representative test numbers:
(I should also note that it has been extremely hot/humid here lately so the water temp has been more like 88-91 degrees F rather my usual setting of 80 - more on this later).

Aug 1st: FC 0, CC 0.5, pH 7.4, CYA 50. -> added 3 jugs bleach (all I had in the garage) to bring it up to 7.5
Aug 2nd: FC 4.5 (loss of 3ppm in one day), CC 0.5, pH 7.2. -> added 8 jugs of bleach to achieve SLAM level. rechecked FC after and it was 28.5. also added more CYA.
Aug 4th: FC 19.5 (loss of 9ppm/2 days = 4.5ppm per day), CC 0.5, didn't check pH due to high FC level. CYA fully dissolved, rechecked level and now 60
Aug 5th: FC 16 (loss of 3.5ppm in 1 day), CC 0.5
***Here I performed the OCLT test and happy to report overnight loss (9pm-5am) was 1ppm, which I take it means I passed the OLCT since a loss of "1ppm or less" per pool school is OK.***
pool water now crystal clear as previously.

My questions are, why the sudden increased loss rate of FC compared to previous weeks/months? should I be worried? does this mean I could have a nitrates problem? should I even be worried about nitrates in the pool? the previous owner of the pool said he had to drain and refill due to nitrates. is it due to the unusally high temps around here which I understand can lead to some acceleration in the breakdown of FC (please correct me if I'm wrong)?
Is my SWG just getting weaker and may need a MA bath and/or I should just increase the % run time?
I have to emphasize that this whole problem started when the pump pressure plummeted and I figure was not circulating the water well which may have caused a slight algae bloom, and I am just now trying to clean that up - however, with consistently low CC levels of 0.5 less and an normal OCLT, I am having a hard time attributing these numbers to organics in the pool.

As always, your input is much appreciated.


Gold Supporter
Jun 13, 2017
Rochester, MN
Hello, neighbor.

I’ve seen somewhat the same this summer. The clouds and rain in June and first half of July meant a lot less chlorine used. Since mid-July, seems to be a lot more needed with next to no rain, warm humid days, and more people in the pool :paddle:. I think it’s just the pattern for this year.


Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
Bedford, TX

There is a huge difference in chlorine consumption between a CYA of 40 and a CYA of 70.. That, and hotter weather and heavy bather load, will use up even more FC..

Not sure what you mean by 15% run time??? But, you have a 33K pool and 40K cell.. In "theory", your cell should produce an increase of about .21 ppm of FC per hour of operation, in a pool of your size .

Most pools use 2 to 4 ppm of FC per day. To add 3 ppm of FC to your pool should take about 14 hours with the cell running at 100% output.


Jim R.


Silver Supporter
Jun 21, 2018
Woodbury, MN
Gonfishin - thanks for your input, makes sense. I do believe weather has been a factor.
Jim R - to clarify re: the run time. the Jandy aquapure SWG, as I understand (it's been a while since I read the manual), you set the chlorine production as a percentage of pump run time. So for example I've had it set at 15% nearly the whole summer, meaning the SWG is running 15% of the time. And I had really no issues with FC chlorine levels until a few weeks ago. the problem is (and correct me if I'm wrong) I have no idea when the SWG is actually operating. there is a button on the panel that says "cell on" and will light up when the SWG is pumping out chlorine but I don't know when it goes on or of.

In any case, from both of your replies it sounds like hot weather, not much rain and higher bather loads may be a reason I have been losing more chlorine than usual. I'm going to increase the SWG production for now and continue to test regularly and add bleach as needed. Will keep ya'll posted. thanks again!