Sundays Off Pools

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What Causes Pools to Turn Green?

04/04/2018What Causes Pools to Turn Green?

Green water might be the bane of every pool owner in history. Other than the pool imploding, green water is probably the most frustrating issue to have to deal with. Everything a pool owner does when maintaining their pool is aimed at preventing the water from turning green. What is it that changes the water so drastically? There is a simple answer, but it is actually more complicated than that. Here are some answers to the question of what causes pools to go green.

The plain simple answer is that it is plant matter, specifically algae, that grows in your pool. A pool acts like a swamp in that the water is stagnant for the most part. Just like a swamp, if a pool if left alone for any extended period of time, organisms will start to grow and multiply. Insects will find it appealing, and bacteria will be everywhere. Lakes do not behave this way because they are moving and have sources of new water to feed them. The more organic matter there is, the more will grow, meaning it will accumulate exponentially.

Algae is one of those plants that will grow in a pool if the conditions are right. It thrives in sunlight and warmer temperatures, and uses carbon dioxide for fuel. Almost everything in your pool, from the debris that gets blown in, to the swimmers in the water, is a source of carbon dioxide or other algae favorites such as nitrates and phosphates.


As mentioned, nitrates are food sources for algae. If your pool has high nitrate levels then you must take action to diminish them. Nitrates are a major part of plant growth, and are heavily used in farming. The nitrates that do not get digested by plants can end up in the groundwater, where they make their way to bodies of water, including swimming pools.


Phosphates are another favorite food for plants like algae. They are often found in detergents, as well as rain and fertilizers. Unfortunately, they are also present in some pool chemicals, so they must be managed well. If you live near a golf courses, your pool is more likely to have an excess of phosphates because of the run-off from the fertilizers used.


Eutrophication is a chemical process for when water has an abundance of a nutrient. When this happens, plant matter will take over. Over a long enough period of time, your pool would turn into a complete swamp through eutrophication if not treated properly. This is the process that you and pool owners around the world are battling every day.

Prevention of Algae

Since algae essentially grows because it is feeding on those nutrients, the best thing to do is to starve it and make the environment less appealing. Nothing to eat means that it cannot produce or develop. Using pool chemicals that balance out the nitrates so that they are neutral will prevent an overabundance of any of them, thereby preventing plant life from growing.


Every pool owner knows about and uses chlorine, but what does it actually do? Chlorine is a chemical mixture that kills every living thing in your pool. However, it works best if the conditions are right. If the water is too acidic, or has pH levels that are too high, then the chlorine will not be as effective. Problems can occur when the conditions for growth for the algae outweigh the conditions for the chlorine to work effectively. It will only be a matter of time before the algae wins the race and your water turns green.

Fighting Green Water

If your water has turned green then you must take drastic action to fight it. First off, adding in chlorine will kill the algae, but that chlorine will be used up, and will not be able to fight new growth. That is why you must “shock” the water with an excess of chlorine to not just kill the living algae, but to leave some to continue fighting. You may have to stay out of the pool for some time until it is safe to go back in the water. Make sure to check your chemical levels to be sure.

A green pool is not inviting, and may even contain bacteria that might be harmful to human health. Now that you know what causes the green, you can more effectively fight it and prevent it from happening in the future.

If you are a Twin Cities resident in need of pool repair services, do not hesitate to contact Sundays Off Pools online or by calling 763-546-1651.