CYA dropping very fast - without water loss

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Toonetown

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Aug 14, 2021
5
Layton, Utah
I just returned from a 10-day vacation. Prior to leaving, I wanted to make sure that all my chemical levels were balanced - and I had everything clean and balanced. I tested the day before I left, and my CYA level was at 75. When I returned, my pool was green (now I get to SLAM!), chlorine was at 0 (and that is clearly why), and CYA was significantly below 30 (probably basically 0 - the lowest my tester will go is 30 - and the water was still clear enough for me to very clearly see the black dot at the bottom - it just looks like clear water).

Is it normal to lose 60+ ppm of CYA over the span of 10 days? The pool was unused the entire time we were gone, and the cover for it was closed. I don't have a leak or any significant water loss - the water level was basically the same when I returned - since it was covered, I didn't even lose much to evaporation.

I'm just wondering what can cause such a drastic drop of CYA (I had a similar situation a month ago when I left for another trip).
 
There is a bacteria that eats CYA and converts it to ammonia. Ammonia eats FC, making CCs. Green pool and zero FC and CYA points to this as a possibility.

Before adding more CYA, add 10ppm of FC via liquid chlorine. Let mix for 15 minutes and test FC again. Let us know what it is.
 
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I just went through the same thing - put in way more CYA than was needed and kept chasing it lower with more CYA until I stumbled on this site looking for help. Did a SLAM and now it's holding. My water was clear but we started to get a small algae bloom in a few spots and must have had this bacteria that eats the CYA. It took a lot of liquid chlorine and requires you to stay on your FC because whatever is in your pool just keeps consuming it...but it sure does feel good when you're done! My pool hasn't looked this good in 2 years. But you have to rule out other issues as well like Ice had mentioned.
 
Toonetown said:
I am actually in the middle of a SLAM - should I let my FC drift back down to 0, add the 10pm as you mentioned and then test again? Or should I continue my SLAM?
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You have to constantly keep your FC at the SLAM levels. You'll have to continuously monitor it - especially in the beginning as whatever you have in your pool is still there and consuming it.

Do not let your FC levels drop below the SLAM levels and make sure you follow the exit criteria very carefully to not stop too early.
 
Toonetown said:
I am actually in the middle of a SLAM - should I let my FC drift back down to 0, add the 10pm as you mentioned and then test again? Or should I continue my SLAM?
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Did you add any cya? If so, dont add anymore - the 10ppm is for when there’s no cya.
If you added cya & you’re cya is now holding & fc is holding longer than 15 minutes keep with slam at the appropriate level for your cya.
Dealing with ammonia is the same as slamming just without feeding the bacteria more cya while you overcome it. It takes alot of chlorine & really staying on top of maintaining slam level
 
Thank you for all the pointers! I will continue and stay on top of my SLAM.

Do I need to monitor and maintain my CYA levels during the SLAM as well? If they drift down quickly over the course of a week or so (like it did while I was gone last week), won’t that change the required FC level needed over the course of the SLAM?
 
Something is amiss. CYA is considered pretty much stable in any pool. You will slowly lose some over time but testing it every 30 days is more than adequate.

You should add enough CYA now (assume 0 in your pool) to get to 30 ppm. Once you dose the pool to get to 30 ppm, don't even test for it......just assume it's in there.

You are really not SLAMming. With 0 CYA, the chlorine loss to UV is just too fast for you to keep up. The 30 CYA will slow your FC loss dramatically and THEN you can restart the SLAM.

I have no idea what happened to your previous CYA. Your scenario really doesn't fit for ammonia in your pool so I would hold back on that to see if you can't first get the SLAM values where they need to be.

There is no reason to retest anything except FC and CC until your SLAM is complete.
 
So - I was assuming my cya levels were fine before slamming - because of the reason you mentions (doesn’t lose much - and I had just tested 10 days before). So, I started my slam under the assumption that I still had 75ppm CYA.

I would add chlorine (liquid - 10%), as prescribed by the pool math app, check again after 30 minutes, and my FC would only be at 1.5-2 ppm. So, I’d add more, and same thing. After adding upwards of 30 gallons of chlorine over 2 days, it finally got to my slam level - but that is where I started questioning my CYA level…because it seemed the chlorine was getting consumed so quickly. So I tested CYA and it was 0.

One note, I do keep my cover closed while slamming to prevent the chlorine from burning off from UV and to keep junk from blowing into it while I’m trying to keep it clean. Maybe I shouldn’t be doing that?
 
If you added chlorine to 10 ppm or greater and ~30 minutes later it was below 5 ppm, you likely had an ammonia issue.

For the CYA test, was there any cloudiness to the mixed sample or was it essentially clear?

Once the FC is holding, add stabilizer for a target of CYA 30 and start your SLAM Process.

Cover should be off so pool can breath and offgas any CC. Pump should be running 24/7 for the SLAM.
 
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For the CYA test, was there any cloudiness to the mixed sample or was it essentially clear?
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I could see no cloudiness at all...filled up the tube all the way (marked as 30ppm on my test kit), and could still see the dot completely clear as though there was nothing in the tube.
Remove the cover completely for the SLAM. Let your CYA buffer the FC to hold back chlorine loss
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Thanks - I will do this then.
 
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