Why Your Chlorine Disappears Overnight

Chlorine that keeps crashing to zero can make you feel like you’re doing everything “by the book” and still losing. We dig into the real-world reasons pools won’t hold free chlorine in summer, when water temps climb, the UV index spikes, and normal dosing routines suddenly stop working. I share what I’ve seen across years in the field, including why a pool can chew through a gallon of liquid chlorine a day and still test empty, even with a CYA level that looks totally fine.

We get practical about cyanuric acid (CYA) from both ends: how low CYA leaves your chlorine exposed to fast sunlight burnoff, and how high CYA can make chlorine less effective even when you’re reading 8 to 9 ppm. I explain the 7.5% free chlorine to CYA rule, what a workable “sweet spot” looks like for outdoor pools, and why you sometimes have to bend your targets to match brutal heat waves.

Then we zoom out to the other big culprits: underdosing based on 10,000-gallon labels, algae that creates a vicious demand cycle, phosphates that feed growth, and certain stain removers or sequestering agents that can wipe out chlorine if you’re not careful. We also cover circulation and variable speed pump runtime, because chemicals can’t work well if they never fully mix and dead zones keep forming.

If you want fewer chlorine surprises and a clearer plan for pool maintenance, listen through and put these checkpoints to work. Subscribe, share this with a pool owner who’s stuck, and leave a review with the problem you’re fighting right now.

We break down why a pool can burn through chlorine even when your numbers look “right,” especially during brutal summer heat and high UV days. We walk through CYA balance, correct dosing, algae pressure, phosphates, product side effects, and why pump runtime can make or break chlorine stability.
• heat and UV driving rapid free chlorine loss even with decent CYA
• low CYA leaving most of a big chlorine dose unprotected
• high CYA making chlorine less effective and pushing higher targets
• using the 7.5% free chlorine to CYA rule
• underdosing from 10,000-gallon label math and why calculators matter
• algae creating a downward cycle of demand, dead algae, and dirty filters
• aggressive algae response, including higher shock levels and filter cleaning
• phosphates as algae food and when to treat based on test results
• stain removers and other products that can zero out chlorine
• longer pump runtime improving circulation and reducing dead spots
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Transcript

Why Chlorine Won’t Hold

SPEAKER_00 1:03

Hey welcome to the Pool Guy Podcast Show. Today's episode I'm going to cover some reasons why a pool may not be holding chlorine. And I'll cover some reasons that are typical and some reasons that are kind of not very typical, but you may run into out there, especially during the summer season. Are you a pool service pro looking to take your business to the next level? Join the pool guy coaching program. Get expert advice, business tips, exclusive content, and get direct support. From me, I'm a 35-year veteran in the industry. Whether you're starting out or scaling up, I've got the tools to help you succeed. Learn more at swimming poollearning.com. One of the misnomers I think is that if you have a good zineric acid level in the pool, 50 to 80 parts per million, let's say, then you shouldn't have any problems with the chlorine zeroing out in the pool. And that's just not simply the case, especially it is summertime when the water temperature is heated up, let's say the water temperature is at 90 degrees, you know, it's 110 degrees out there, the UV index is at 10, and the sun is just beating down on the pool all day long. If you're adding just the regular amount of chlorine, you always add to the pool, even if you have a good zinaric acid level, it's going to burn off much faster than you think. I had someone email me one day, and they had a 30,000-gallon pool neighbor in Texas, and it was middle summer, really hot, you high UV index,

Heat And UV Burn Off Chlorine

SPEAKER_00 2:28

and he said he was adding a gallon of liquid chlorine to his pool every day. He was buying the chlorine at a pool store, adding a gallon a day, and it was still zeroing out every day. And I told him that's not unusual with the weather conditions. By the way, his cyanaric acid level was fine. I think it was like at 50 parts per million, and it's just one of those things where even if you have everything by the book with the variables of the weather, it really doesn't work out that way. So you have to kind of bend a little bit to the environment. And then when you're in one of these high summer heat waves with really high temperatures, high UV index, yes, the cyanuric acid will protect the chlorine to a point, but it's still gonna burn off a lot faster than if it was 80 degrees, and the water temperatures in the 70s, you're not gonna have as much burnoff as you would have when it's 110 with a high UV index. So keep that in mind that a lot of environmental factors like the weather really do affect the free chlorine level rapidly, and you may be adding a lot more chlorine than you normally would be adding, and kind of scratching your head. It's not really a mystery, it's just that it's just really blazing hot out there, and the chlorine, even though it has cyanaric acid, is not protecting it. Since I'm talking about cyaneric acid, a lot of times the reason why the chlorine is not holding in the pool is that there's too little cyanaric acid. Now, I know that I was afraid of having too much cyanaric acid in their pools, you know, 100, 200 parts per million, but there's an opposite spectrum where customers have maybe 20 or 30 parts per million in the pool, and that's not effective either. Because the old rule of thumb is that for every part per million of cyaneric acid, it'll protect the chlorine part per million level. So let's say for instance you have 30 parts per million of cyaneric acid in the pool, you're only gonna protect the chlorine up to 30 parts up to three parts per million, I should say. So if you're adding a gallon of liquid chlorine, most of that is gonna be unprotected because it only protects it up to three parts

When CYA Is Too Low

SPEAKER_00 4:34

per million. And if you raise the pool to ten parts per million, within like three or four hours, seven parts per million will burn off out of the pool because it's not being protected by the low cyanaric acid level. Now, if you raise the cyaneric acid to 50 parts per million, then you're gonna protect that chlorine level up to five parts a million. If you raise it up to 80, you'll protect it up to eight parts a million. But when you start getting into the 100, 150, 200, it has kind of the opposite effect that you would think it would have. It actually will require you to add more chlorine to the pool to maintain to maintain that pool. And that's why a lot of times the pool's not zeroing out, you're at eight or nine parts per million, but you still have algae forming in the pool because the cyaneric acid level is too high in the pool. And that's because the formula that Bob Lowry came up with is showing, is actually telling in the fact that too much cyanaric acid actually interferes with the chlorine's ability to work properly in the pool. And this formula is that the free chlorine level needs to be at 7.5% of the cyaneric acid level in the pool. So if your cyaneric acid level is at 100 parts per million, you're gonna need about 8 parts per million of free chlorine for the chlorine

When CYA Is Too High

SPEAKER_00 5:43

to be effective. So there is an opposite end of the spectrum. If you have too little cyaneric acid, it's not gonna protect the chlorine level or chlorine in the pool. If you have too much cyaneric acid, it's gonna make the chlorine ineffective in the pool. So there's a sweet spot. I think in most cases, 50 to 80 parts per million is a sweet spot for me for pools that I know there's a good amount of cyaneric acid to weather these hot weather storms and high UV index days, but not too much cyanaric acid where the chlorine is ineffective, or you need to add a lot more chlorine to the pool to make it effective. Kind of go back a little bit to the first example with the person adding a gallon of liquid chlorine to his 30,000 gallon pool in Texas and not holding effectively. And one of the mistakes that you can make during the season is that you're adding chlorine to a level that's not high enough to make it effective in the pool. Kind of keep that 7.5% formula in mind here. A lot of people will just add chlorine because they look at the bag and it says one pound of shock, you know, add this bag to raise it to three parts per million, whatever, and 10,000 gallons of water. So they'll just add one bag of shock. But if you have a 30,000 gallon pool, you technically need to add three of those bags for it to be effective because you have to multiply the 10,000 gallon baseline that all chem all chemicals are based off of by the size of your pool. So if your pool is 20,000

Stop Underdosing Your Pool

SPEAKER_00 7:06

gallons, adding one pound of calhypo to that pool is not going to move effectively raise the chlorine to where you want it because you'd have to actually add two bags of calipo or two one-pound bags of calipo to your pool. So basically, there's a multiplier with chlorine to make it effective. Also, besides the cyaneric acid to free chlorine ratio, there's also the gallons of your pool water versus the 10,000 gallon dosing on the bag or liquid chlorine. So if you're going to effectively dose your pool, I really suggest using an app like the poolcalculator.com. You go to my website, swimmingpoollearning.com, scroll down to the middle, and you're gonna see the pool calculator app there. And this will allow you to put in the amount of gallons you have in your pool, and then it'll actually give you the exact dose of chemical cal hypo, liquid chlorine, dichlor, whatever you're putting in the pool, to the point where it's going to be an effective level. So again, you can't underdose and expect chlorine to hold in the pool because if you have a 20,000 gallon pool, the chlorine is at zero, and you want to raise it to five parts per million, you may need more than just one pound of cal hypo in that pool to be effective. So underdosing is a big problem in the in the summertime. And I would say that you always want to err on too much chlorine instead of too little chlorine when you're adding it to the pool. Another problem is when the pool starts to get algae in the pool, it becomes a big problem, especially in the summertime. Now it's not just the problem of algae in the pool, it's a problem of kind of this perpetual kind of downward cycle that happens with algae in the pool. I'll explain it like this. If the pool has algae forming in the pool and you kind of treat it but don't take it too seriously, what happens is the algae starts to bloom and get even worse in the pool. What this happened happens next is you have to shock the pool with a lot more chlorine, and then at that point the filter is getting a lot of dead algae in the filter, and that means that the filter is not running correctly, and so even if you shock the pool without cleaning the filter now, you're still gonna have algae in the pool, and the pool may still zero out because the algae is growing more rapidly than you can fight it, and so the algae becomes like a perpetual downward

Algae Creates A Chlorine Spiral

SPEAKER_00 9:53

cycle here. And there's been many customers I've taken over their pool, like in you know, July, late late July, and they tell me that they couldn't use their pool all summer yet because they're fighting this algae problem that just won't go away. And so there's a lot of different things that go into this. When you have algae in the pool, there's factors again, maybe too little cyaneric acid, the chlorine's not staying in the pool, maybe too much cyanaric acid, the chlorine's not being effective in the pool, and then the algae itself kind of perpetuates and grows and blooms and kind of gets worse and worse as the day goes by. Even in a 24-hour period, the pool can go from just having you know some mustard algae on the deep end wall to the next day being completely full-bloomed and turning kind of greenish. And I've seen this happen before. So algae is something serious, not a joke, and algae is a big contributor to the pool zeroing out in chlorine. And so my suggestion, of course, is that you attack the algae aggressively if you have some spots in a deep end. Bring the chlorine to 30 or 40 parts per million right away. Use that pool calculator app to dose the pool to that level. You also want to make sure that you use an enhancer and algicide. So a pool RX is great, adding boris to 50 parts per million. Of course, using a phosphate remover would help as well. When you have algae in the pool, you may want to do a full phosphate treatment or just a maintenance phosphate treatment depending on the phosphate level in the pool. You can get phosphate test strips on Amazon and you can see how high your phosphate level is in the pool. And phosphates are a problem, especially in the season in the pool, so you want to make sure you treat the pool for that. And if the allergy was particularly bad, you also want to clean the filter or backwash the filter to make sure the filter is clean because, again, it's kind of a vicious cycle where the allergy gets in the filter. Filter is now dirty, and the chemicals you're adding to the pool won't be as effective because of the dirty pool filter. And I kind of glossed over the phosphates right there, and I'll probably go a little deeper here so that you get an idea that phosphates are a big problem and a big contributor to zero chlorine. Now, phosphates get into the water in a lot of ways, it can blow in from the wind, could be some products you're using in the pool or household products getting into the pool, whatever the source. Once you have a phosphate level of over 500 parts per billion, in my opinion, you want to treat that phosphate problem right away because it just grows and gets worse because it's a really good food source for algae. And by eliminating that one food source, there's other food sources for algae, but that's the main food source, in my opinion. And if you remove the phosphates out of the water, the algae doesn't have that food source anymore, and therefore the algae won't bloom or grow to a great extent. So if your pool, if you notice that the chlorine is zeroing out, you may not

Phosphates Feed Chlorine Loss

SPEAKER_00 12:48

have algae per se, it may just be a little murky, maybe not as clear as you would like the pool to be. Go ahead and do a phosphate test. You can get the phosphate test strips again on Amazon or your pool store, put them in there, check the phosphate level. And if it's reading anything over 500 parts per billion, I would do a phosphate treatment. Now there's different products you can use to do the phosphate treatment. If you wanted to really knock it out, the PR 10,000 by Arenda is a great phosphate treatment, but you have to do the full treatment, and you have to also clean the filter. If you just have a small amount of phosphates, you can get away with using phosphory and putting it in the skimmer and using a maintenance dose, and that eventually will destroy or actually eliminate the phosphates, just to stay in the water. But sometimes a full phosphate treatment with the PR10,000 is necessary, and that means that you have to clean the filter more than likely, also as well as part of the treatment, but it'll definitely eliminate the phosphates in the water, and you should be able to maintain a good chlorine level after that. Now I mentioned that there are some products you may be using that introduce phosphates to the water. There are some pool products that have it, some tile soaps and things like that do have phosphates that will be added to the pool water. There's also some products like the stain removers, like the Biodex AquaDEX 50, or some other sequestering agents that actually will destroy the chlor level in the pool. So be aware that a lot of these products will lower the chlorine level in the pool or even zero out the pool. I had this pool where it was it had some staining that was really bad, and I just figured more is better. You know, this is in the early days when I was doing this, and more is actually not better. So I use a whole bunch of stain remover in the pool, and it zeroed out the chlor level to where you know after three weeks I could not get a chlorine reading in this pool. I mean, I realized that I overdosed it

Products That Crash Chlorine

SPEAKER_00 14:42

with the stain remover, and it just probably had a you know negative chorin reading in here at this point. So I had a really hard time bringing it back. So really don't overdo it or don't overdose the pool with the stain remover. If you use a quart of Aqua Dex of Biodex Aqua Dex 50, you should be fine. Just realize that the chlorine level is gonna suffer when you use any kind of stain removing product, Hasa Super Stain out, anything like that will affect the chlorine level in the pool. And be just be aware of that fact that some products actually will lower the chlorine level in the pool, and if you're not aware of it, it it is something that does happen. And this one's not quite as obvious, but it's I think it's worth mentioning, and that is not running the pool long enough for the chemicals to circulate and be effective in the pool. A lot of this has to do with variable speed pumps. A lot of customers don't like running their pool on a higher speed, which is okay, it's perfectly fine. You just have to run the pool longer. So there are some customers that think that you know four hours at 1800 RPMs and three hours at 2400, 2400 RPMs is fine for their 20,000 gallon pool. A lot of times circulation is the key. I've serviced some pretty large pools that were heavily used over the years that were heated during the summertime to like 85 you know 88 degrees. And really one of the most effective things that I did for these pools was run these pools 24 hours a day, like a commercial pool basically. And so if you have a variable speed pump, very easy to do, you can run it for like 23 and a half hours with

Pump Runtime And Real Circulation

SPEAKER_00 16:17

it, you know, on higher speeds, medium speeds, whatever. But as long as that pool is running 24 hours a day, it's gonna circulate the water really effectively, circulate the chemicals, not too many dead areas in the pool where algae can form, and that long runtime is the key for pools that are heavy, heavily used. So if you have pools on your route that you know is that are used really heavily during the summer, really boost that runtime up. And I'm not kidding, maybe 24 hours will do the trick in most cases. It just run those pools as long as you can, and that really is half the battle. The longer the pools are run, in my opinion, the less water quality issues you have, and the less problems you have with chlorine steering out in the pool. Of course, all these other factors I mentioned, but the runtime is a factor that should also be noted and addressed. And the longer you run the pool, the better off you're gonna be, definitely in the summertime as well. If you're looking for other podcasts, you can find those by going to my website, swingingprolearning.com, on the podcast, on the banner, there's a podcast icon. Click on that for a drop down menu of over 1900 podcasts there. And if you're interested in the coaching program that I offer, you can learn more at poolguidecoaching.com. Thanks for listening to this podcast. Have you guys your week? God bless.