CYA Issues


New member
Aug 20, 2020
Napa, CA
Hi everyone. After divorce, I decided to do the pool care in my new, downsized home. All was well, and I was quite proud of myself for almost two years...

Until my pool got mustard algae...

I used a number of products that were recommended to me... YellowClear, a phosphate remover... My floater seemed to be going through tabs quickly...

Then I decided to a couple of shock treatments with granular chlorine...

I now know about CYA... and that tabs and granular chlorine are loaded with the stuff... And my CYA level is over 100. (I have a locally-owned pool supply place in town with honest guys who know their stuff. He had me bring a water sample in and tested the CYA. He said I had chlorine lock and needed to drain & refill; that there are products that claim to reduce CYA but they do not consistently work and are expensive.) He said the combination of tabs & granular chlorine sent my CYA levels high, high, high. (Why don't they discuss CYA on the instruction panel of the granular chlorine?)

I did some research on chlorine lock and ran across articles that said it was a bogus term and to shock the pool. Interestingly, it lead me to this site.

"According to the experts over at, it’s safe to say that chlorine lock just isn’t a real issue. In fact, there is no proof that an excessive amount of cyanuric acid will cause the chlorine “lock.” Don’t be confused by the two. Chlorine demand is real and solvable by adding enough chlorine to your pool until it starts to read on your liquid test kit or test strips. Chlorine lock is a false issue, but keep in mind that some pool companies and experts will use these two phrases interchangeably. Take comfort in the fact that whichever the phrase used, both are solvable by feeding the beast."

To add insult to injury, my daughter didn't turn the water fill valve completely closed and my pool filled to the absolute brim. And being on city water, I really do not want to drain 1/2 of my 10,000 pool.

So I'm looking for options. I will admit, I got completely lost in the CYA chemistry discussion in The Deep End (clever forum title.)

I'm throwing out an idea, which I'm sure will go down in flames, but I'm still going to put it out there..

> First off, I've removed the floater and purchased liquid chlorine from Home Depot.
> I put a gallon of liquid chlorine in last night and ran the filter all night. My chlorine reading was high this am and low in the afternoon. (No surprise, based on what I've read here.)
> Can I continue to use liquid chlorine at night to keep the mustard algae at bay and keep adding water as it evaporates? If I'm running the filter overnight, I should be getting the water cycled, right?
> Do I need to add liquid chlorine at shock levels? Or just enough that it registers in the am so I know it's been doing its job overnight?
> I also have phosphate remover; will this help by adding some in the evening with the chlorine, or perhaps in the am and let the filter run an extra hour to dispurse?


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Bronze Supporter
Jun 11, 2018
Verona, MO
Welcome to TFP! :)
I don't doubt your pool guy is honest and is knowledgeable. Sounds like more than most stores. But you still need your own test kit unless they are using a titration kit like the TF100 or Taylor K2006. Even then, when slamming you will be checking every couple of hours so lots of trips to store. I use the TF100 with speed stir, link in my signature.
The experts need to see a full set of test results from an approved kit to be able to make good recommendations.

Please also add your pool information to your signature, similar to mine. Helps the experts help you and saves you typing in each time you start a thread or ask a question. Settings/signature

I suggest adding enough liquid chlorine to reach 5ppm daily while waiting on your test kit to arrive, then post a full set of results. Morning or night additions don't really make much difference. With high CYA, it is being tied up and you are not going to kill the algae, merely hopefully hold it off a bit. You can't really slam it to kill it without knowing exactly what the CYA level is. See chlorine/cya chart in my signature.

I suggest reading the ABC's of pool water chemistry, link in my sig also.

Good luck!
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