FC low after attempt at SLAM

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B

barbc2000

Active member
May 16, 2022
31
East Longmeadow MA
Pool Size
26700
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-45
FC has been low and it was suggested I have algae and should SLAM. I used the Pool Math calculator and it said to raise my FC from 0.5 to 24 I need to add a little over 6 gallons of 10% liquid chlorine.

Yesterday I added 4 gallons because I didn't want to overshoot. Tested an hour later and FC went from 0.5 to 1.0.
Added another 4 gallons. Tested an hour later. FC 1.5
Added another 2 gallons (that is all I had left thinking I only needed 6!). Tested FC 1.0

We did turn on the SWG because the manual said FC 1-3 was okay. So ran it all night.

Pool water is crystal clear the whole time. Pool water is still clear this morning and this is the first morning it doesn't even have anything on the bottom that needs to be vacuumed.
This morning results:
FC 0.5
CC 0.5
TA 80
CYA 60
pH 7.0? Not sure, the color is orange. Doesn't match anything. So either lower than 6.8 or between 6.8-7.2.

Why is my FC so low after adding so much chlorine? The pool math calculator recommended to add 6 gallons and I added 10 gallons yesterday and barely raised the FC level!
 
Either something is chewing through the chlorine, or the chlorine you're adding is not the strength you think it is. What is the date on the chlorine bottles?
 
Yep, unless it was some sort of bad batch of chlorine (not likely) or Walmart threw it in an oven (also not likely though I wouldn't put it past them), the chlorine is probably strong and fine to be as expected.

Which leaves something eating through it. Maybe another chemical - was anything else added to the pool, any algaecides or anything like that? Anything unexpected get into the water since it last held? Has it held FC at all earlier this season?
 
IceShadow said:
Yep, unless it was some sort of bad batch of chlorine (not likely) or Walmart threw it in an oven (also not likely though I wouldn't put it past them), the chlorine is probably strong and fine to be as expected.

Which leaves something eating through it. Maybe another chemical - was anything else added to the pool, any algaecides or anything like that? Anything unexpected get into the water since it last held? Has it held FC at all earlier this season?
Click to expand...
We got the pool 8/20/21 when we bought the house. I think the previous owner did nothing to it that summer. It had leaves and worms in the deep end. We cleaned it out. Brought a water sample to Leslie pools. They kept telling us we had metals and needed to get rid of the metals because it interfered with all the other results. We took out the ladder which had some rust on it and added something to get rid of the metals. I don't think we addressed any other problems; like no chlorine, and finally decided to close the pool for the winter. We bought a kit to close and called it a season.

We opened it a few weeks ago. There was some visible algae and I think we added one bottle of algaecide. Since then we have added a lot of chlorine. We also brought the CYA up from 0 to 60.

I brought a water sample to Namco today. Told him the whole story. He suggested we may have to drain a foot at a time and add new water. I forget what he said exactly but I think he was saying the water was saturated and we have added too much stuff.

Still only 0.5 for FC. I tested 10 minutes after adding 2 lbs of chlorine granules (that is all I had left).
 
Looking at your pool math log, you likely have either algae or ammonia (even if you can't see it). We can help you correct either one.

Best thing to perform is 1) Ammonia Test and 2) an Overnight Chlorine Loss Test (OCLT).

To test for ammonia, add enough liquid chlorine to the pool to raise your FC to 10ppm. Test FC after 30 minutes. If it is down to 0-2, you likely have ammonia. Then follow this:

Ammonia - Further Reading

www.troublefreepool.com www.troublefreepool.com

To do an OCLT tomorrow night:
www.troublefreepool.com

Perform the Overnight FC Loss Test (OCLT)

The Overnight Chlorine Loss Test tells if the SLAM is finished or if you need to start a SLAM. This article says how to do the OCLT.
www.troublefreepool.com www.troublefreepool.com

Between your test results, and these two test, we will be able to help you get your pool to look like this:

How Clear is TFP Clear? Let's See (Pics Only Please).

How clear should my water be? A very common question. Here is mine on a calm morning where I could finally vacuum. Yes I know, I'm still manually vacuuming. It took me 7 years to finally get an SWG, I'm working on a robot. :) So now let's see your water clarity.
www.troublefreepool.com www.troublefreepool.com
 

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What method are you using to measure FC and CC. Are you using FAS-DPD? If yes, can you describe how you’re performing the test.

Let’s just make sure the correct testing procedure is being used to rule that out.
 
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Reactions: PoolStored
PoolStored said:
Looking at your pool math log, you likely have either algae or ammonia (even if you can't see it). We can help you correct either one.

Best thing to perform is 1) Ammonia Test and 2) an Overnight Chlorine Loss Test (OCLT).

To test for ammonia, add enough liquid chlorine to the pool to raise your FC to 10ppm. Test FC after 30 minutes. If it is down to 0-2, you likely have ammonia. Then follow this:

Ammonia - Further Reading

www.troublefreepool.com www.troublefreepool.com

To do an OCLT tomorrow night:
www.troublefreepool.com

Perform the Overnight FC Loss Test (OCLT)

The Overnight Chlorine Loss Test tells if the SLAM is finished or if you need to start a SLAM. This article says how to do the OCLT.
www.troublefreepool.com www.troublefreepool.com

Between your test results, and these two test, we will be able to help you get your pool to look like this:

How Clear is TFP Clear? Let's See (Pics Only Please).

How clear should my water be? A very common question. Here is mine on a calm morning where I could finally vacuum. Yes I know, I'm still manually vacuuming. It took me 7 years to finally get an SWG, I'm working on a robot. :) So now let's see your water clarity.
www.troublefreepool.com www.troublefreepool.com
Click to expand...
Haha...so I added 10 gallons of liquid chlorine and the highest FC reading was 1.5ppm. If I could get the FC to 10 I wouldn't have a problem. That's my problem!
Katygreys said:
What method are you using to measure FC and CC. Are you using FAS-DPD? If yes, can you describe how you’re performing the test.

Let’s just make sure the correct testing procedure is being used to rule that out.
Click to expand...
I have the TF-100 Pro kit. I use the "chlorine only" vial. Fill to 10ml mark. Add a generous scoop of the powder and stir with the magic stirrer. The powder turns it very light pink. Only 1 drop of the liquid will make it clear.
 
Ammonia doesn't seem to make sense with low CC and CYA holding. But something is consuming the FC. That or the test reagents are bad.

Have you tried scrubbing out the test vial? Wash/scrub it out well, rinse well, then rinse with the sample water once before doing the test. Maybe something in the vial is interfering?

Have you tried the color comparison test that comes with the TFT-100 to see if that shows no FC as well?
 
IceShadow said:
Ammonia doesn't seem to make sense with low CC and CYA holding. But something is consuming the FC. That or the test reagents are bad.

Have you tried scrubbing out the test vial? Wash/scrub it out well, rinse well, then rinse with the sample water once before doing the test. Maybe something in the vial is interfering?

Have you tried the color comparison test that comes with the TFT-100 to see if that shows no FC as well?
Click to expand...
I read the ammonia information, and I agree, my numbers don't fit that problem.

I have tried the color comparison test to check. It confirms the same low FC.
 
Yes, you need to keep doing this until FC holds. Adding the chlorine consumes the Ammonia. It may take repeated attempts. The point of the process is that you do not know how much ammonia you have, therefore you don't know how much chlorine it will take to get rid of the ammonia. Dosing in 10 ppm allows you to consume it without "overshooting."

Follow the process. When your FC finally holds, then switch to the SLAM process.
 
FC will eventually overtake whatever is in the water if you add enough of it.

The danger is we can't say how much that will be because we don't know what's in the water. Something is there that is consuming FC. It's also being consumed by the FC, but we don't know how much of it is there.

So - you can either follow the protocol for Ammonia (which is to keep adding 10 FC at a time and retesting, repeat until FC holds) or you can exchange the water out. I have to imagine that the costs for the water exchange might be less, depending on what it costs to fill the pool. If you do drain, I would do a No-Drain Water Exchange to avoid potentially shifting the liner. Here's info on that: Draining - Further Reading .

Which way to go depends on your cost of water and how lucky you feel. It could be the next 10ppm you put in finishes off what's in there and you hold chlorine fine. Or it could be dozens and dozens of chlorine jugs. Changing out the water is going to be more certain of a cost you can know, but will cost more than adding jugs if the next one would have been the last. Ultimately it's your call on which way to go.
 

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