Chlorine level won’t drop

Irecently converted my pool to a salt cell. After getting the pool chemistry balanced and having the salt cell run for a couple of weeks, I decided to try the super-chlorinate feature on the salt cell. The chlorine level stayed very high for a several days, so I thought I had created a chlorine lock situation and put some chlorine shock in the pool to see if that would break the chlorine lock. Now, five days later, my chlorine reading is still off the charts. My local pool store’s test computer recommends 80-150 CYA. The Pentair instructions recommend 30-50 CYA. Is it reasonable to assume that’s my problem?

If so, what’s the best way to reduce the CYA level. Can it be done without draining/refilling?

I sure appreciate any help.

Hmmm.  You did not post any of your numbers from your pool water.  What is the CYA (stabilizer) level in your water, and what is your chlorine level?  If your chlorine is high, did you turn the chlorine generator off and wait a few days to watch the levels (hopefully go down)?  Lastly, are you testing your own water?   Some pool store water testing can be unreliable at best.  Get a good, professional test kit like the Taylor Water Test Kit FAS-DPD - K-2006.  They are easy and fun to use (especially if you liked the chemistry set you might have had as a kid) and are very accurate - and dramatically better than test strips.

Saltwater pools can get by with lower CYA levels (and lower chlorine levels, too).

I’m doing both with my Taylor test kit and the local pool store. In both my test and the store test, the CYA is 90. My test kit doesn’t go high enough to measure the chlorine, but the store test says 13. The cell has been off for a couple of days now and again, I can’t measure the chlorine until it drops quite a bit. When I use the OTO tester, the sample turns orange after only a couple of drops.

Ph is a little high at 8.1, but I’ve read that’s because the chlorine is so high. All the other numbers are in range.

Hello Quickstep,

I agree with InyoRich, without more information it’s going to be hard to help you definitively. Can you tell us what type of pool do you have, vinyl, plaster, fiberglass, etc,?

We need a complete set of test readings please. PH, Total Alkalinity and Calcium Hardness levels are very important in pools using a SWG. And the type of pool surface dictates what those numbers should be.

Your Taylor test kit won’t accurately read the chlorine level because you are using a DPD chlorine kit like the Taylor K2005. The Taylor K2006 kit uses FAS powder with the DPD reagent to test chlorine. It will read chlorine levels as high as 50 ppm.

That said, I will answer what I can.

From the limited information you have given, my best guess is that you super chlorinated the water and after super chlorination you didn’t turn the SWG completely off until the chlorine level dropped back down to the normal chlorine level (which would be 4 to 6 ppm for water with 90 ppm CYA).
It is not reasonable to assume that CYA is your problem.
First, don’tlisten to the pool store about CYA levels. CYA should neverreach 150 ppm. It shouldn’t reach 100 ppm.  And I must respectfully disagree with InyoRich about SWG pools getting by with less CYA than pools sanitized with chlorine. SWG pools require more CYA. The ideal CYA level for SWG pools is 70-80 ppm with 90 ppm being the maximum acceptable range. The 30-50 ppm range that the manual recommends would be the ideal level for a pool sanitized with chlorine. I would contact Pentair and ask them to clarify.

There are only two ways that I know or have heard of to lower CYA levels: 1. Drain water and replace with fresh water. 2. I have read about a reverse osmosis process that can lower CYA levels, but it is extremely expensive and I’ve never found anyone offering that service.

The PH level is high. It ideally should be between 7.2 and 7.8. A reading of 8 is ok if the Total Alkalinity and calcium hardness readings are where they should be.

If you can provide the additional information about your pool type and a complete set of test readings I will be happy to provide any additional help that I can.


Following up.

The pool is a 15,000 gallon plaster pool.

Hardness is 258

Alkalinity is 116

Salt is 3500

i appreciate any further info, but it looks like I’m going to have to lower the level and re-fill to get the CYA down

Thank you.

I wouldn’t recommend draining water at this time. A CYA level of 90 is within the maximum acceptable range for pools with a SWG. The only thing the higher CYA level means is the SWG wil need to generate 1 additional ppm of chlorine than if the level was 70. And the CYA level doesn’t have anything to do with the high chlorine level that won’t come down. Have you turned the SWG off completely until the chlorine level comes down?

**1.**Your TA level is high. It should be between 70 - 80 ppm. This is why the PH readings are high. High PH levels can lead to scaling.

Lowering the PH will also lower Total Alkalinity. Use Muriatic acid to lower the PH level by .4 ppm. This will lower the PH to 7.6. It will take 22 oz. of 2O Baum (31.45%) Muriatic acid to lower PH in your pool by .4 ppm.

Slowly pour the acid in the deep end in front of a return jet with the pump running. And don’tinhale the fumes. After about 2 hours, with the pump running, check the PH level and the TA level. If the levels have dropped, wait a day and check it agan. Because of the high TA level you will probably find that the PH has drifted back up. If it has, continue this process until the PH is in the 7.4 -7.6 range and the TA is in the 70 - 80 ppm range.

**2.**The calcium hardness level is low. Plaster pools with a SWG should have at least 350 ppm calcium hardness. Low calcium levels in a plaster pool will cause the water to leach calcium from the plaster, pitting or etching it.

13 pounds of calcium chloride will raise the hardness level by 94 ppm in a 15000 gal pool. Dissolve small amounts of the calcium in a 5 gallon bucket full of water and slowly pour it in the deep end in front of a return jet with the pump running. You can do this at the same time you are doing the PH.


Thanks for that info. Yes, the SWG has been off for several days.

You’re welcome.

Until the chlorine level lowers significantly, I cannot measure it. After just a few drops when I use the OTO tester, the sample becomes orange.

Until the chlorine level lowers significantly, I cannot measure it. After just a few drops when I use the OTO tester, the sample becomes orange word wipe


Please start a fresh post describing your issue. Please include the type of pool surface and the latest test readings for TA, PH, CH and CYA ( you’ve already said that you can’t get a chlorine reading). This way we can help you individually.


I think I may have figured out what my problem is.

I tried a different test kit and got a reading within range. I know we don’t trust test strips, but a test strip verified the results of the second test kit.

Does OTO reagent go bad?

During my original tests using OTO, the sample would turn orange after just a few drops.

Hi Quickstep,

All reagents have a shelf life. So yes they can go bad. If that is the case then you have seen firsthand the importance of reliable accurate test readings. I’ll state again that the most reliable and accurate chlorine readings are obtained using a FAS/DPD test. Please consider purchasing a test kit that uses FAS/DPD, like the Taylor K2006 or the TF-100