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Why Do Pools Turn Green?

Why do pools turn green? If you are asking "why do pools turn green" then right away I can tell that you would benefit from reading some of the entry level pool ownership articles that I have written. This is not simply because you are asking a dumb question, which you are not, but because you are interested in knowing more about exactly what is happening in the water. This curiosity is the backbone behind successfully owning and caring for a pool. This question about why pool water turns green, however, is the entry point into a very deep conversation. There are two ways to answer this question which are the short way, and the long way. Starting with the short way:

Pool water turns green because plant matter is growing in the water. Moss grows on a rock, grass grows in your front yard, and algae grows in your pool

If you are happy enough with the short answer, then you can take the pool chemistry crash course to expand on how to balance your pool water from a technical perspective. You may also like to learn about chlorine free pools and the risks associated with operating a pool without a chemical sanitizer. If you are new to owning a pool and want to start with the basics of terminology, equipment and basic pool maintenance then you should definitely start with the new pool owner 101.

What Causes Pool Water To Go Green?

What causes pool water to go green? Perhaps a better way to look at this question is to ask "what stops pool water from turning green?" If left alone to let nature take over, every swimming pool would turn green. It is actually a constant battle to keep the green at bay in a pool, and this is because a pool is more like a swamp than a lake. The main difference between these two is that a lake will have water feeding into it, where a swamp would most typically be stagnant water. Swimming pools, if left to nature, are stagnant and the water would quickly be taken over by plant life, bacteria, insects, and eventually the local wildlife. Without a water source feeding nutrients and oxygen into the body of water (or a pump and filter circulating and cleaning the water), the rate of organic matter growth in the water will grow exponentially. Most importantly, without a sanitizer in the water to restrict the growth of algae, it will grow so fast that a pool can go from blue to green in a matter of hours.

Algae is a living creature that is in your pool. If you provide the right (wrong) set of conditions then algae has the potential to grow very quickly. Algae likes warm temperatures and most like sunlight since photosynthesis is how algae produces sugars for energy. In addition to this, algae needs carbon dioxide and a food source. So what is a food source for algae? Just about everything in your pool actually. From the bathers themselves to the wind blown contaminants in your pool, pretty much anything that gets in the water will feed algae. Most notably in terms of algae food in pools is nitrates and phosphates.

algae in pool water

Nitrates In Pools - Nitrates are chemical compounds that include oxygen and nitrogen. Nitrates act as a food source for algae in swimming pools and can be notoriously hard to deal with if you have a problem with high nitrate levels in your water. Plants, like algae, use nitrogen to produce amino acids which aids in the production of proteins. Nitrates are a critical component in agriculture and are used extensively in farming, however when nitrates make their way into a body of water like your swimming pool they enable advanced rates of algae growth. Since nitrates are water soluble, any nitrates not absorbed by plants will eventually make its way into the groundwater system. Over time these nitrate levels add up and can be found migrating to places like water supply sources, and of course, swimming pools.

Phosphates In Pools - Phosphates are chemical compounds that include oxygen and phosphorus. Phosphates are also a food for algae and are commonly found in detergents, rural water supplies, municipal water supplies, rain water and fertilizers. Phosphates are also found in many common pool chemicals such as stain removers and metal treatment additives. Swimming pools located close to golf courses (who extensively fertilize and maintain their landscapes) are notorious for having extremely high phosphate levels.

What Is Eutrophication? - Eutrophication is the name for the process by which water has too much available nutrient in it, such that plant matter and algae growth will eventually take over. If given enough time, a swimming pool left to nature would undergo eutrophication where the water would turn green, and eventually become a swamp like environment, and given enough time and the right conditions, eventually a grassy, plant filled field.

It is this process of eutrophication that you are fighting in your swimming pool. In order to stop the process you need to limit organic matter growth. In order to limit the organic matter growth, you need to interrupt the life cycle somehow. One of the best ways to accomplish this is to effectively starve out the algae in your pool. If there is no food for algae then it will not be able to grow very fast, or at all, which is ultimately the goal.

Why Is Algae Such A Problem In Swimming Pools?

algae growth in pool water Up until now in this article, this all sounds very straight forward. Algae grows from eating nitrates and phosphates - so what is the problem? Well, it is important to note that this information is very watered down. The actual science behind nitrates and pool water, and even more so with phosphates, is much more complex than it may seem. For example, "phosphates" are not a singular thing. Phosphates are a dynamic, complex series of chemical compounds, and can exist in many different forms such as organic phosphates and inorganic phosphates, of which there are many subclassifications such as orthophosphates and condensed phosphates. If you have "phosphates" in your pool, this could actually be many different kinds, and that is only the tip of the iceberg!

So knowing there are different kinds of phosphates, it is next important to understand that there are also different kinds of algae. Every type of algae will be able to feed on phosphates, but the rate of growth (and ideal growth conditions) changes from one type of algae to another. For example, there is a type of algae that grows aggressively at 1000 parts per billion of phosphates, and you you might try to lower your phosphate level to 100 parts per billion or less if you have a problem with these in your pool. Unfortunately there is a different form of algae that prefers phosphate levels at 100 ppb instead of 1000 ppb and so you can end up with an algae problem of a different kind. The problem of nitrates and phosphates fueling algae growth in pools is a very complicated one for any person who is not a chemist that specializes in this field.

Understanding how algae grows in water, which compounds feed and promote algae growth, and how phosphates and nitrates get into the pool is only one part of the science behind why pool water turns green. The other major part of the equation is the effect that chemical sanitizer, like chlorine, has on limiting algae growth in your pool.

Does Chlorine Kill Algae In Swimming Pools?

chlorine kills algae So now you understand that algae is growing plant matter in your swimming pool that is feeding off of chlorophyll from the sun, as well as carbon dioxide, phosphates and nitrates in the pool water. So how does chlorine fit into all of this? Chlorine kills algae. Actually, chlorine kills pretty much everything that is trying to live in your pool, including the algae. In a perfect sense, this is all you would need to know. In the real world there are a lot of factors that effect how well chlorine can do its job. High cyanuric acid levels in the pool, or high pH levels, can both cause chlorine to become less effective, or not effective at all at killing algae (and other things like bacteria).

In addition to chlorine not always being able to kill algae, there is also the condition that if factors for algae growth are just right, you will have trouble maintaining your chlorine levels. The rate at which algae grows when supplied with the things it needs, most specifically a food source like nitrates of phosphates, is greater than the rate at which chlorine can kill the algae. More accurately, there is usually not enough chlorine reserve available in an average residential pool to combat a sudden algae bloom.

How To Fix Green Pool Water - When conditions are perfect the algae will grow incredibly fast. When the available free chlorine in the water becomes depleted, the rate of algae growth increases even more since it is no longer being killed off. This is how a pool can go from clean to cloudy to green all within a very short period of time. Even if you add more chlorine in this situation, it is often immediately used up killing the existing algae. Now it becomes a race between how fast algae will grow and how fast you can add chlorine to your pool. This is why when you deal with an algal bloom in your pool it is important to shock it aggressively enough as to overcome the chlorine deficit in the water, and establish a chlorine reserve. Many pool owners immediately turn to algicide when their pool turns green, and of course there is a place for algicide but it should not be a first line of defense. Chlorine is an algicide. That is precisely one of the reasons that we use chlorine in pools - one of four reasons specifically.

If you have green water in your pool then establishing a chlorine reserve is paramount to resolving the problem, as are limiting any possible food sources like biofilm, nitrates and phosphates. Equally as important is making sure that your cyanuric acid level is in the correct range of 30 to 50 parts per million, and that your pH is 7.5.

In total, the process of your pool water turning green involves the presence of algae as well as food for algae in the form of carbon dioxide, nitrates and phosphates. In addition to this, the free chlorine in the pool must be depleted to zero, or inhibited from killing the algae by unbalanced variables like pH or stabilizer in the water. Together, if you have enough food for algae, and the right environmental conditions, as well as a lack of algae killing chlorine available in the water, algae will grow and your pool water will turn green.

Can You Run A Pool Without Chlorine?

10 Minute Pool Chemistry Crash Course

7 Good Pool Maintenance Habits You Should Adopt

New pool owner guide

Is salt water bad for pools?

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