My SWG/Tab method

I have an Aquarite system in my 20k pool/spa combo that works quite well. It has it's issues, such as a Ph level that rises faster than I believe a standard pool does, and also a whitish deposit that appears in the grout at the waterline that is impervious to all attempts at removal (but dissapears when wet) but overall I would never ever go back. However towards the end of the season I noticed that my CYA levels had dropped to below 30ppm from the 50-60ppm 6 months before. Now I know in theory this shouldn't be possible but, I'm telling you the honest truth, that CYA do gradually go down in my pool at a rate that cannot be explained by splashout alone. Anyhow I decided to shut down the salt cell and start using tabs just long enough to get the CYA back up to the 70-80ppm range. By adding 2 tabs per week I can easily maintain around 4ppm FC. I figured it this way because it would extend the life of the cell by having it "rest" for a few months in the winter and also if I'm going to be adding stabilizer why not chlorinate at the same time, killing 2 birds, rather than spend money on just CYA? As a side benefit, the pH does not rise as quickly, both from the tabs inherently low ph and the fact that the SWG is off, so for now I use a little less acid to control ph and less baking soda to control dropping TA. I get the tabs cheap through a pool guy I know, so I'm not so against tabs as I used to be. They deffinately have their uses. I figure that CYA will be right within the next couple of months and then it's back to the Aquarite. Now the big question is how is CYA being consumed (albeit slowly) in my pool, in contradiction to everything I've read on the subject?


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Apr 1, 2007
Sebring, Florida
Hi, Frank,

Welcome to the forum.....interesting post. All your thoughts make sense to me although I question that the tabs will bring your CYA up enough with what seems like your judicious use of them. You may have to add some more CYA to get where you need to be.

How are you testing your CYA level? I can think of no reason for your CYA to recede other than testing vagaries.
Has your salt level remained constant or has that decreased also? There really would be no explanation for the CYA to decrease if the salt remains constant but if you have needed to add salt also then there is an easy explanation. . . you have a leak somewhere.
You also did not state what type of filter you have. If you have a filter you backwash this would cause a gradual decrease in both CYA and salt levels.

Dave also raises an excellent point about your testing method since CYA is the most subjective test.

One other possible explanation would be if you had a period of heavy rains. Often this can cause the hydrostatic relief valve to open and allow groundwater into the pool which can dilute the chemicals but your salt would also go down in this case.
I am using the Taylor "black dot" test. I've had luck with it in the past, when an addition of stabilizer resulted in the correct expected change in the test. Also, I compared it once to the pool stores test and they were very close if I remember correctly. I'd say in a year my pool loses 30-40ppm of CYA give or take. I can't imagine that close to half of the pool water is splashed out in a year. The waters always crystal clear, and I live on the west coast so I never close the pool. I started out using 3 tabs per week for awhile, but then my FC was around 10ppm which seemed excessive really. No need for that high a residual in 50-60 degree temps. I've already seen a 10ppm rise in CYA since I started this. I think by February or March I'll be set.

Oh yeah, my salt levels have remained pretty constant, I maybe add the equivalent of a bag of salt per year, and I think that's about right for splashing out? I have a cartridge filter, so no backwashing is going on. Very little rain here in Central California.
I might have figured out where your CYA is 'disappearing'! Your last post gave me a clue. What is the temperature of your water sample when you get the low CYA readings? The CYA test slows down quite a bit at temps below 70 degrees and can take quite a long time for completion, perhaps up to 10 minutes. If read before that it will read low. Next time take your sample indoors and let it warm up to above 70 degrees before you test and see what happens! (I see this all the time at work when people bring in cold samples and I let them sit before I test their water and the readings are normal. Otherwise the CYA will read low.) Also, be sure to shake the mixing vial for 1 minute and let it sit for 3-5 minutes then give it a quick shake again before doing the reading in the view tube.
Well I tried bringing the water sample up to room temperature before CYA testings, and yes, the reading is higher by between 10-20ppm, which seems a considerable variance. Although it seems even after taking this into consideration, my cya still seems to fall gradually throughout the year, I'll just have to keep an eye on it to see if this trend continues regardless of warming up cold water samples.


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May 7, 2007
Silver Spring, MD
The CYA test is +-10 to +-15 in addition to it's sensitivity to temperature. CYA levels often fall slowly through the season because of water replacement, typically from back washing and/or splash out.