Pool is clear but overnight test still fails

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zhenya

Gold Supporter
Sep 15, 2013
46
Los Angeles, CA
I've noticed some algae on the bottom of my pool few days ago. It wasn't much and the water was still clear all the way to the bottom. As advised here I started a SLAM process. I actually miscalculated a bit and instead of advised 20ppm SLMA level took it all the way to 30ppm.

After two days of SLAMing the water is absolutely clear there is nothing on any walls or bottom, but my overnight test still fails. Last night between 8pm and 7am it dropped by 5.5 ppm.

Any advice on what else I could do or what could be causing this?
 
Check all nooks and crannies. Check under ladders, inside light niches, and inside the skimmer and see if you find any algae lurking there continuing to use up chlorine. Then brush 'em out and get them floating so they can be killed.

What is your CC reading?
 
Thanks. I took out the light and cleaned inside (it was actually clean) cleaned steps, cleaned everything like you said. Haven't checked inside autofill yet. that's a good idea actually.

my CC reading is 0.5 and it's been at 1 before I started SLAM
 
Chlorine demand and OCLT tell us when things are wrong with the water. I had to SLAM this year because,of a stupid mistake but the water was always sparkles clear. I just noticed a higher chlorine demand, proved a problem with the OCLT and started the SLAM. Even with beautiful water it took me 4 days to eradicate whatever was in there.

Trust your testing. If you fail OCLT there is a reason.
 
Thanks, Tim, that's encouraging.

One thing that came to mind. Could agae be lurking in my filter? Should I clean the filter. Mine is DE and backwashing it a bit tricky as you never know how much DE to add. Pressure on filter shows that it's fairly clean.
 
zhenya said:
Thanks, Tim, that's encouraging.

One thing that came to mind. Could agae be lurking in my filter? Should I clean the filter. Mine is DE and backwashing it a bit tricky as you never know how much DE to add. Pressure on filter shows that it's fairly clean.
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I don't think algae can live in the filter due to zero sunlight. Besides, you're passing SLAM water through it 24/7. It probably has better exposure than some parts of the pool.

Hang in there .........

Dom
 
tim5055 said:
Chlorine demand and OCLT tell us when things are wrong with the water. I had to SLAM this year because,of a stupid mistake but the water was always sparkles clear. I just noticed a higher chlorine demand, proved a problem with the OCLT and started the SLAM. Even with beautiful water it took me 4 days to eradicate whatever was in there.

Trust your testing. If you fail OCLT there is a reason.
Click to expand...

+2 I had a couple of times this summer with the same thing. Higher chlorine usage and had to do a couple of SLAMS. I never noticed a difference in my water but something was brewing.
 

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Chrissykr said:
+2 I had a couple of times this summer with the same thing. Higher chlorine usage and had to do a couple of SLAMS. I never noticed a difference in my water but something was brewing.
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Yea, I know,what caused mine. 4 day weekend out of town and I had turned off the Stenner pump to do some work before I left town....

4 days in the hot SC sun with no chlorine.......
 
domct203 said:
I don't think algae can live in the filter due to zero sunlight. Besides, you're passing SLAM water through it 24/7. It probably has better exposure than some parts of the pool.
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It's true that the algae won't grow in a filter, but that doesn't mean that chlorine isn't still oxidizing the dead algae. If a filter is full of debris that chlorine can react with, it is best to backwash or clean (as appropriate) the filter.
 
Also remember that there could be other things in the filter that will consume chlorine. I kept having a mystery chlorine demand this summer, ended up opening up my DE filter to find the DE was saturated with some sort of oily substance, I don't know what it was or where it came from, but there was a LOT of it.
 
zhenya said:
After two days of SLAMing the water is absolutely clear there is nothing on any walls or bottom, but my overnight test still fails. Last night between 8pm and 7am it dropped by 5.5 ppm.
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Q: What test kit are you using and what sample size did you use to do the overnight? Do you have one of the recommended test kits?
Q: What concentration are you using and have you checked the time stamp on the bottles? Sometimes, the calculations could be off because the % strength is not accurate.
Q: Have you verified the size of your swimming pool?

Any variables in these items could cause an error. Also saw that you had an 11 hour time lapse between testing? Maybe shorten the time to 1/3 (8 hours). If all these are fine, then something is definitely lurking in that water.
 
I am using Taylor Kit. Using 10ml sample size
My chlorine is 8.25% purchased at Costco
I verified the size of my pool several times, but how would it be relevant. I am comparing results between two different tests using the same pool, same chemical, etc.

It's now been 5 days and still the same trouble with overnight test. I am loosing patience not to mention kids are ready to kill me because I don't let them swim. I am going to do a full filter clean on Wed so hopefully it solves the problem. I don't know what else can possibly cause chlorine disappearing so quickly. The pool is perfectly clean all around.

Thank you all for your suggestions. Would love to hear more ideas of what I might be doing wrong.
 
Well zhenya, here's a thought for you to consider because it happened to me personally. Since I didn't notice your current CYA anywhere on this thread, it may be relevant to you as well. Earlier this season I kept failing my OCLT. So much so that I went through almost TWO bottles of R-0871. I was going nuts! Why? Because as you noted in post #1, I too was over-shooting my SLAM FC by waaaaaay too much. So much in-fact that even though I had no apparent loss to algae, some of my excessive FC had to drop in relation to the CYA. If my SLAM FC was 24, I had my FC at night as high as 35 or more. When it comes to the OCLT, more is not always better. :)

So unless I missed it somewhere, we don't know what your actual CYA is right now. But whatever it is, definitely make sure you are not exceeding the corresponding SLAM/Shock FC level for that CYA as listed on the Pool School - Chlorine / CYA Chart. If you go too high on your FC, then you might lose more than 1ppm of FC. Example: If your CYA is 50, then don't go over an FC of 20. Anyways, once I realized I was simply going way too high, the very next night I let my FC come down to the regular SLAM level, and passed the very next morning. Maybe that will help you. Good luck.
 
Exactly. Me too. Drove me nuts. Not sure if the FC will lower fast enough for you before the sun goes down on the west coast, but if not, definitely let it come back down and try it tomorrow at the proper level. Say, you might want to double-check the CYA chart unless that was a typo above. A CYA of 30 calls for a SLAM/Shock of only 12. An FC of 16 is for the CYA of 40. :) Either way you've been a bit high, so maybe that will help you pass next time. :) Good luck!
 
And of course ... if you still happen to fail the OCLT later, try not to be too discouraged. Not all bad stuff in the water is visible to the naked eye. It just means your bleach is still cleaning the water for you and the kids. You'll get it. But I hope the correct FC level does the trick for you.
 
As for frustrated swimmers, keep in mind that it is safe to swim with FC up to shock level, as long as the water is clear enough for decent visibility. Of course, it sounds as though you have been running well above shock level for your CYA much of the time... Maybe this is another incentive to stay closer to the recommended level. Let it drift just below shock level, let kids swim, boost it back up a bit after they're out.
 

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