(Updated Title) Sudden complete FC loss due calcium chloride

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tech135797531

Well-known member
Jun 14, 2019
53
FL
Ladies and gents, I'm stumped. After treating my fiberglass pool for iron with ascorbic acid 5 weeks ago, I had mostly re-balanced it. FC was steady at 4.5-5 for 2 weeks, alkalinity was 70, PH 7.6 CYA 60. The only thing that was low was calcium so I raised it over the course of the last 48 hours from 250 to 380 (I added 12ish lbs). I tested the FC 2 days ago and it was almost 5 and after adding the last 6lbs of calcium last night, there is 0 FC. Nothing.

There are no signs of algae, water is about 65 degrees with outside temps 75 during the day, 60 at night (N FL), the filter is clean with no signs of algae. I know algae can hide, but there is nothing to have caused such a rapid drop in under 48 hours. And the aquarite 940 had been running at 20%. No one used the pool. I did not think balancing the calcium could affect FC. Is there any logical explanation for what just happened? I'll dump a bunch of liquid chlorine after the sun goes down tonight but I am a bit freaked out.

Thank you!
 
Add the chlorine now. Use enough to raise the FC to your target value.

Adding calcium should not effect FC levels. So just get chlorine in the water now.
 
Hey tech !!! It’s to be expected. I lifted the pertinent parts from pool school for ya

After AA treatment.
  • You can start to bring up your chlorine. You want to do this slowly. It will take a lot of chlorine - preferably using liquid chlorine only at this time, and try to take it up to your minimum chlorine for your cya according to the Chlorine/CYA Chart.
  • As you slowly raise the chlorine, watch for stains starting to form.
  • If you see stains, make sure the ph is no higher than 7.2, and add more sequestering agent. You should see the stain lift in a couple of hours.
  • Do not raise to SLAM FC levels! Do not do a SLAMfor at least 2 weeks!
  • Make sure you keep your ph low for (7.2) for a week or two - it won't be hard because the ascorbic acid and sequestering agent will help it stay low.
  • Once your chlorine starts to hold, it means that you have used up the ascorbic acid in the water and it will be easy to rebalance the pool back to recommended levels.
 
@Newdude , thank you but I don't think that's it. I went through that entire process a month ago. It took 2 weeks but I got the Chlorine to be steady at 4.5-5ppm and it held there for over 2 weeks. The only thing I've done over the last 48 hours is add 12lbs of calcium increaser, 2lbs per bucket of water, 6lbs per day, 2 days back to back. That's it. FC was 5 before and absolute 0 now. No signs of algae, no swimming, no temp/sunshine changes, everything else is balanced. Even if I had some algae that I can't see, it shouldn't drop like this.

@mknauss , doing it now. Any idea what could have caused it? I don't mind feeling like an idiot but I'm trying to know what I did to be an idiot :)
 
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If you added calcium chloride to increase hardness, calcium chloride additions will crash your FC. I’ve had multiple experiences with it. The phenomenon seems to go unexplained here on TFP but it happens.
 
tech135797531 said:
Ladies and gents, I'm stumped. After treating my fiberglass pool for iron with ascorbic acid 5 weeks ago, I had mostly re-balanced it. FC was steady at 4.5-5 for 2 weeks, alkalinity was 70, PH 7.6 CYA 60. The only thing that was low was calcium so I raised it over the course of the last 48 hours from 250 to 380 (I added 12ish lbs). I tested the FC 2 days ago and it was almost 5 and after adding the last 6lbs of calcium last night, there is 0 FC. Nothing.

There are no signs of algae, water is about 65 degrees with outside temps 75 during the day, 60 at night (N FL), the filter is clean with no signs of algae. I know algae can hide, but there is nothing to have caused such a rapid drop in under 48 hours. And the aquarite 940 had been running at 20%. No one used the pool. I did not think balancing the calcium could affect FC. Is there any logical explanation for what just happened? I'll dump a bunch of liquid chlorine after the sun goes down tonight but I am a bit freaked out.

Thank you!
Click to expand...
What product did you use to increase the CH?
 
@JoyfulNoise - I used SunCoast Calcium Hardness which uses Calcium Chloride Compound 100%

@revitup - I've seen some of the reports here that adding calcium chloride caused FC drop but every single response seems to be that it's not correlated. But see what I wrote below, it has to be.

I have just gone from curious and surprised to really concerned. Yesterday I added at 1pm 48oz of liquid chlorine 10.5% which should have bumped me to 5ppm. 8 hours later, I tested 2.5ppm. I added another 24oz which should have bumped me back to 5. Indeed, at midnight I tested 5. This morning at 8.30am I tested 4.6. All good. What's not good is the fact that now, 6.5 hrs later, I'm back to 0.

There are no traces of algae, water temp 66, air temp 76, AQ940 is running at 22% which is high to begin with for where it should be this time of the year.

I am at a loss. I've lost 8ppm in less than 24 hours for no apparent reason other than adding 12lbs of Calcium Chloride Compound 100%...and it's not supposed to cause this. Any thoughts?
 
Last edited:
mknauss said:
Do an Overnight Chlorine Loss Test
Click to expand...
As I mentioned in my post above - at midnight I was around 5 (maybe a touch higher, tough to tell under a light bulb), at 8.30am I was at 4.6. So that was fine. What happened yesterday and then again today between 8.30am and 3pm is the issue. I even cleaned the salt cell last night to ensure proper operation during the day. And today has been mostly cloudy.
 

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I’m convinced the FC loss is directly related to the calcium chloride addition. I experience a FC loss every time I add calcium chloride. The good news is it does not last, at least in my experience. Just keep on top of the FC level and in a short while it will hold.
 
I just want to summarize the facts:

1. the pool was balanced and stabilized for over 2 weeks following an ascorbic acid treatment 5 weeks ago (where I followed the steps described on the forum for the 4th year in a row).
2. The only thing that was out of balance was the calcium. I added 12lbs, 6bls one day, 6lbs the next day.
3. The day after the 2nd treatment I checked the FC and it had dropped from 5 to 0 while everything else continued to stay balanced.
4. I added 48oz of liquid chlorine yesterday at 1pm. it dropped from 5 to 2.5 by night time. I added another 24oz. I tested 5ppm for FC at midnight and FC only dropped 0.5 overnight. It then completely (from 4.6 to 0) today between 8.30am and 3 pm on a cloudy day. I added another 48oz at 3.30. At 5pm I tested 4.5ppm. I'll retest at 7.30pm and then tomorrow morning.

Ph = 7.6
Alkalinity = 70
CYA = 60
Calcium is 380 (raisedfrom 250)

Salt cell stayed steady at 22% during this time, air temp = 75, water temp =67, today was cloudy, no one used the pool. I cleaned the filter again, no signs of algae, I cleaned the salt cell last night as well.

If I knew nothing about pools, based on these facts, the only logical explanation is that the calcium chloride did it. I'll keep adding chlorine and report back. I don't know what else to do. But if someone else has any thoughts, I'd love to hear them.
 
Last edited:
aussieta said:
there are too many smart people on TFP with chemistry knowledge
if there was a reason for calcium to drop FC they would have found it by now
Click to expand...
One would think. But one would also think that if a direct correlation is demonstrated under very careful and documented test conditions, multiple times, of calcium chloride addition, exclusive of any other chemical manipulation, resulting in an abrupt and significant FC loss, TFP experts and proponents might have to face the reality of a phenomenon they may not yet understand.
 
by coincidence i have added 10lb (4kg) of calcium chloride over the last few days
on the 20th december my fc was 9.0ppm
we have had extreme temps the last few days, 38c (100f), uv index extreme
with high bather load
today measured 6.5ppm fc
no change to swg settings
so i am not seeing anything unusual
 
aussieta said:
by coincidence i have added 10lb (4kg) of calcium chloride over the last few days
on the 20th december my fc was 9.0ppm
we have had extreme temps the last few days, 38c (100f), uv index extreme
with high bather load
today measured 6.5ppm fc
no change to swg settings
so i am not seeing anything unusual
Click to expand...
Would have been more interesting to know what your FC was immediately before, and then immediately after, the calcium chloride addition.
 

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