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Why Is My Pool Water Green?

Why is my pool water green
If you are a pool owner wondering why your pool water has turned green, despite your best efforts, you will probably find the answer to your question contained on this page. There are only so many reasons why your pool could or would turn green and if you start looking at your water chemistry numbers the answer should probably jump right out at you. Within the pool and spa industry there is somewhat of a running joke. If I asked another pool professional how their water chemistry is they would respond with numerical figures for free chlorine, total alkalinity, pH...things to that nature. When you ask a new pool owner about their water chemistry they almost universally answer by saying "the water chemistry is good". Firstly, the water chemistry is not "good" or we would probably not be talking about your green pool water and how to fix it. Further to this, and this is the point here, pool owners fail to recognize the importance of water chemistry readings.


When a pool professional inquires about how your water chemistry is what they are expecting to hear in return is a systematic list of all of your chemical parameters including numerical values. The origin of this problem is from pool test strips, the lowest common denominator for pool water testing, because they tend to use basic terms like "low, high, and ideal range" when in reality you should know these ranges already off by heart, and you should determine for yourself whether a number is good, bad, or otherwise. Here is the minimum information that you must come armed with if you want to have a discussion with a water chemistry expert as to why your pool water is green:


Total Alkalinity - 60 to 180 ppm (100 to 120 ppm optimal range)

pH - 7.2 to 7.8 (7.4 to 7.6 optimal range)

Calcium Hardness - 150 to 1000 ppm (200 to 400 ppm optimal range)

Free Chlorine - 1 to 5 ppm (2 to 4 ppm optimal range)

Combined Chlorine - 0 to 1 ppm (0 ppm optimal range)

Cyanuric Acid (CYA) - 20 to 80 ppm (30 to 50 ppm ideal range)

Phosphates - 0 to 1000 ppb (under 500 ppb optimal range)

Copper - 0 ppm

Iron - 0 ppm


Yes that is a lot of numbers to remember. Fortunately for you we live in a day and age where everyone carries a supercomputer in their pocket 24/7 that is connected to a collective database of the entire knowledge of mankind. Your grandparents just had to know this stuff. You really don't have an excuse, except for that everyone is so busy these days that who has time to learn pool chemistry! Make it easy on yourself and just take a picture of your screen right now and you will have most of the information that you need to maintain your pool chemicals all in one picture. Of course you can also just bookmark this page, or my whole website, and come back here every time you have a question about your pool or your water.



First steps when your pool water is green - There is definitely an order of operations that, for the most part, will apply to all pools with green water. If you are a new pool owner, or struggling to understand how to stop your pool from turning green, then the very first thing that you need to know if that you need chlorine in your water. I think all pool owners know by now that chlorine is how you treat swimming pool water however we need to start at the beginning here, and that most certainly is the chlorine level in your water. If you do not have chlorine, or stated another way, you measure zero parts per million of free chlorine in your pool water...well I think we have found the problem. Pool water needs chlorine. it is not enough to add some, so there "must" be some in there, right? You need to add chlorine to the extent that you are able to build, and hold, a residual value in the water. This ability to build and hold a residual value is one of the unique factors that makes chlorine the gold standard for pool water care. If you can, at any time, measure your chlorine level and it is equal to zero then this is the reason for your green water. Without a sanitizer in the water, your pool immediately begins to grow bacteria and organic matter. How long it takes to turn green depends on your filtraion quality, water temperature and amount of sun exposure, but zero free chlorine means green pool water with certainty.





Without chlorine, a few hours in some pools and a day or two in others before the water predictably turns green. If you added chlorine but the test still measures the free chlorine count as zero parts per million, then this means you have "no chlorine" even though we both know you do have some. You put it in yourself yesterday! That chlorine that you added found bacteria and organic matter and attacked it, sanitizing and oxidizing it in the process. You now have used chlorine, also called chloramines or combined chlorine, but you no longer have free chlorine. With no chlorine free, and able to kill stuff in the water, your water has turned green again. If you add chlorine and the next day it is all gone when you test the water, you either have algae and bacteria in the water that uses up the chlorine as fast as you add it, or you are missing a critical component of chlorine maintenance in pools which is the cyanuric acid level.


Green Pool Water & CYA - CYA, cyanuric acid, also known as pool conditioner and water stabilizer is the chemical you add to your pool to prevent the UV from the sun from degrading your chlorine levels. Without CYA the sun would burn off all of your chlorine in your pool, every day. If you are having trouble with green water in your pool there are two ways that CYA could be at the root of the problem. If you do not have any CYA, or a number too far below the minimum threshold of 30 ppm, then the UV from the sun will degrade too much of your chlorine, and you will struggle to hold a residual value in the water. If you find every day that all of your chlorine is gone from your pool it could be your CYA is too low. On the other end of the spectrum, having a CYA level that is too high is one of the single most common reasons that new pool owners will have their water turn green. This will be especially confusing to pool owners, because the water test will still show that you have some chlorine but the water is acting as though you have none (turning green). This happens if your CYA levels get higher than 80 to 100 ppm. Chlorine pucks and other forms of stabilized chlorine products contain CYA and so every time you add them to your pool your CYA levels are increasing. When the level hits around 100 ppm you enter chlorine lock, where the chlorine is not able to perform properly in the water. It is still there if you test for it, but your water turned green because it is not able to function. You might sometimes see a reference to CYA levels being okay up to 150 ppm but this presumes that you will also increase your free chlorine values to 5% of the CYA values. A CYA value of 150 ppm would then technically require a minimum free chlorine level of 7.5 ppm which is higher than most people keep their residential pools. In short, keep CYA between 30 to 50 ppm and if your levels exceed 80 to 100 ppm you should partially drain and refill with fresh water that does not contain CYA. Set your levels to 30 to 50 ppm and then only use non-stabilized chlorine in your pool like liquid chlorine or chlorine derived from salt water.


Green Pool Water From High pH Levels - The next logical cause for green pool water, after you have established that you have a free chlorine residual, and that you have CYA in the range of 30 to 50 ppm, and definitely not above 80 ppm, you should consider your pH levels. The optimal pH for your chlorine to be functional is actually a much more acidic level than your body would be comfortable soaking in. Specifically chlorine would be way more effective in your water at acidity levels closer to beer or black coffee, which is around 5.0 or so. You might think that sounds low, especially when you look at your pH and see it at 8.0 or above (which is very common, especially in salt water pools), but you might not realize the pH scale is logarithmic. A pH of 5.0 is one thousand times more acidic than a pH of 8.0. Want to go for a dip? Unfortunately people do not tolerate acidic pH like that very well at all, and so we keep the pH closer to what is comfortable for us, ideally 7.4 to 7.6 but, this is a compromise with the chlorine effectiveness. Yes, when kept at 7.4 to 7.6 your chlorine can function and do its job and you can be comfortable at the same time. However this is the very upper end range for where chlorine can work. You just assume that you put chlorine in water and Bob's your uncle...but it is a sensitive balancing act of chemical interactions which arrives at clean and safe water. By the time you pH climbs to 8.0, which it can do easily, readily and all the time, your chlorine is barely keeping up. By 8.2+ which represents the maximum range for many pH testing methods the chlorine is almost completely inactive in the water. You can test for it, and it is there, but it can not kill bacteria or inhibit organic growth. It is worth noting that pH in particular is challenging to test for in your water. Test strips for free chlorine levels are fairly effective for a quick check, but the pH much less so. The color scale for measuring pH with test strips is vague at best, and each person will view and interpret the colors a little differently. It would be helpful for pool owners to know and understand that this limitation with pH testing is one of the main reasons that you should invest in a more advanced home test kit like this complete Taylor pool & spa test kit.


Green Pool Water From Phosphates - Phosphates are a concern for modern day swimming pools. Truly phosphates are a concern for pretty much all bodies of water, both man made and naturally occurring. When you read the news each summer about aggressive and sometimes dangerous algae blooms happening on beaches and coastlines, this is most likely a result of phosphates finding their way into the ecosystem. This is a product of industry and you would typically find phosphates used in manufacturing processes, but also in consumer products like soaps and detergents, fertilizers, even some algicides contain phosphates which can accumulate to levels that are problematic in pools and spas. Phosphates are a common problem in rural water supplies like wells that absorb ground water in agricultural areas where phosphates are used heavily as a nutrient base for plant growth. When they make their way into your swimming pool they are a huge problem for all of the same reasons that the agriculture industry likes to use them. Phosphates promote voracious organic matter growth. As described to me, phosphates are like a super flood for organic matter growth and algae. Given the right mix of phosphates, nitrates, organic matter, UV and water temperature you can see explosive growth in matter that is enough to choke the life from open waterways. A stagnant body of water like a pool with a modest filtration system is simply not able to keep up with plant matter that grows this quickly. This is why we measure phosphates in parts per billion instead of the normal parts per million that we use for so many chemicals in pool water. Above even 500 ppb phosphates will be requiring you to use more chlorine than you normally would. At 1000 to 2000 ppb the chlorine can not keep up with the growth of organic matter, and your pool will be green despite daily chlorine treatments. If you have elevated phosphate levels in your pool you must use a phosphate remover product like natural chemistry phosphate remover. If you rely on a water lab for your water tests you often need to ask specifically to have phosphates tested as this individual test is more expensive than the regular chlorine, alkalinity and pH testing you have done. You probably would still not have to pay for the test, just that many do not include it unless you ask about it. You can also test for phosphates pretty easily with these easy phosphate test strips however note that these only read up to 1000 parts per billion.


Green Pool Water From Copper - You have tested and verified a free chlorine level between 30 to 50 ppm. You have a CYA level between 30 to 50 ppm. Your alkalinity was set to 120 or so (alkalinity before pH) and then pH adjusted to 7.4 to 7.6. You tested for phosphates and have zero, or well less than 500 ppb, or have used phosphate remover to get less than 500 ppb, and your pool water is still green. You might be getting frustrated thinking none of the answers are solving your problem, but the nature of solving swimming pool problems often boils down to a process of elimination. It is helpful that we have eliminated all of the things above this point as the potential cause for your green water. If you have worked to establish your chemicals as listed in the above paragraphs, and then you have been patient for at least 48 hours to let the pool chemistry fully adjust to the changes that you have made. In short, give the pool a day or two to respond to your improved chemical parameters, such that the water has a chance to start to clear up. Assuming you did all of that and still the water is green, the list of potential causes grows shorter. We need to look towards less common causes for your green water. One of those less common but entirely possible causes for unexplained green pool water is oxidized copper. If you have copper in your pool system anywhere that is dissolving into solution it can turn green when it is oxidized by your chlorine. Chlorine is an oxidizer, and copper turns green when it oxidizes. You could have copper pipes in your system as many older pools did. You could also be putting the copper in yourself from something like a mineral system, or copper ionizer, or from algicides that contain copper, or from a failing and corroding pool heat exchanger...there is actually a lot of ways you could end up with copper in solution in your water, and when you add a bunch of chlorine your pool turns green suddenly right before your eyes. It has a unique coloration, in that oxidized copper in pool water will be bright, emerald green, but the water should still be largely clear. Green from organic matter growth normally makes the water cloudy and turbid, but oxidizing copper will look like clear, lime jello pool water. Just like when this happened with the world watching at the Rio Olympic Games when hydrogen peroxide was added manually to the diving tank pools and the pool instantly turned a bright green color. Have your water tested for copper or buy some copper test strips for testing at home. Your value should be zero. Examine the chemicals and peripheral items you have installed to see if there is a source of copper you are unaware of. Treatment of metals in pool water is a complicated subject with a few different ways to approach long term treatment such as sequestering agents or chelating agents to hold metals in solution and prevent them from oxidizing, but then you could also approach the problem from the direction of a flocculant or coagulent, like alum to drop the metals out of solution such that they can be controlled and removed from the pool completely. The best solution for you depends on what your levels are, and how these metals are getting into you pool water.


Green Pool Water From Poor Filtration - Water filtration in swimming pools is one of the major fundamental subjects that is simply approached incorrectly by the vast majority of pool owners. It is terribly common to hear pool owners say they measure filtration schedule in "hours per day" of pump run time. When communicating with other pool owners this can be very misleading if you fail to appreciate that every pool is different, and even two identical equipment systems will operate differently on two different pools. When you account for the fact that some pools are small and some are big, and some filtration pumps are small and some are massive, we would be ignoring a ton of important data to just say "run your pump eight hours per day". There is only one correct way to measure pool filtration, and that is simply to turn over the volume of your pool through your filtration system three times every 24 hours. This results in about 95% of all the water in your pool being filtered at least one time. Which is the goal. Your goal is to filter all of the water in your pool every day. This is the established standard to which you must strive to achieve if you want to filter your pool water "properly". The only other option you could argue is that you should turn over the water in your pool four times every day, which results in about 98% of all of the water in your pool being filtered every day. Anything other than this exact approach is ignoring important technical data. So why do people do this? The answer is shockingly simple. To measure filtration properly you need to know how much water your pool system is moving. To know this you would need a flow meter. Commercial pools have flow meters because they need to actually know this information and follow the rules for safety and to pass regular inspections. If you had a flow meter you could calculate your pool turnover rate as well, which would be great because not only could you know if you are actually filtering the correct amount of water, but you will also know everything you need to know in order to dial in the filtration (and savings) with a variable speed pump. For more information about how the industry ended up like this, and how variable speed pump requirements by the department of energy will see more residential pools with flow meters moving forward, you can read this article about why pools need flow meters. If you have green water in your pool then you should abandon any discussion of reduced filtration schedules. Filtration schedules apply to clean and clear pools. If you have a green pool water problem that you are trying to solve then you would be well advised to run your filtration pump 24/7 until the pool water is sparkling clear. If you have only been running your pump a few hours per day while trying to resolve a green water problem then this certainly is working against your efforts to clear the water.


Pool chemistry is overwhelming if someone just throws a chemistry textbook at your head, or when you have no idea which parameters you should be looking at in the first place. Once you start to learn a little more about the process of elimination, and the numbers behind the colors you read on your test strips, a lot of the mystery is removed from the equation. If you found this article helpful then I am very sure that I have more that you will find helpful as well. Be sure to subscribe to my YouTube channel, and you can keep up with my regular blogging about swimming pools on this page: The Swimming Pool Steve Blog.


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Swimming Pool Steve

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