What Causes Algae in a Swimming Pool?

Michael Keenan


Pool Troubleshooting

Today I want to take a look at what causes algae in a swimming pool. Algae is a common problem in pools and hot tubs but it is easy to get rid of. It is also easy to prevent …. when you understand the causes.

While a dip in the pool can be the most incredible thing on a hot summer day, maintaining the pool is also one of the most important aspects of pool sanitation. You don’t want to get infections or fall sick from using a dirty pool.

Algae are one of the most common pool contaminants. Not only can it cause skin infections, it also multiplies very fast and can mess up the pool filter system irreparably.

But if you want to avoid algae growth in your pool, you should know why it happens in the first place. The following are some of the common reasons algae grow in swimming pools:

what causes algae in a pool

Quick Answer:

Algae is a common problem in swimming pools and hot tubs, but it’s easy to get rid of as long as you understand the causes.

Contamination, poor water circulation, insufficient chlorine or chemical treatment, and an upset pH balance are all main causes of algae buildup.

To prevent algae in a swimming pool, make sure your pool has proper water circulation, good sanitation, and check the pH level regularly with a pool test kit. With proper maintenance and care, you can enjoy a clean swimming pool all summer long!

Contamination and the Right Conditions for Algae to Grow

To begin with, algae is introduced in a pool if there is contamination from the outside. Algae spores reproduce within themselves, which is what allows algae to multiply in a pool so quickly.

In other words, if you spot the first trace of algae in the pool, you are already late to take action but you should not delay treating the pool any longer.

In a day, the algae can multiply if the conditions are right and you might find a thin film of algae or grease on the surface of the water.

Algae spores can be introduced into the pool for a variety of reasons. It can be blown into the pool by the wind or by rain. If you happen to use contaminated equipment in the pool, that might be a factor as well.

Even if someone took a swim in the ocean or in a pond and did not clean their swimming trunks properly before entering the pool, they could also be a carrier of the algae spores.

If algae spores have been introduced into the water and the conditions are right, they will multiply like nobody’s business, so you must act fast!

The usual perception about contaminants like algae is that they prosper in cool and dark parts of the pool. However, it is quite the opposite. Algae require sunlight to thrive and multiply.

If the weather is warm and there is ample sunlight (looking at all you pool owners in sunny California), then the conditions are absolutely perfect for algae growth in the pool if the spores have already been introduced.

The water also needs to be stagnant for algae to grow. Additionally, if there are nitrates and carbon dioxide in the pool, then there is a greater chance that the algae will proliferate.

severe algae problem in pool

Main Causes of Algae Build Up in Pools

We’ve looked at the conditions algae loves to thrive. Now let’s look at some of the common causes of algae problems in a pool. Things you need to avoid.

Poor Water Circulation

As mentioned above, one of the prime conditions for algae to thrive is if the water is stagnant. Pool water is basically stagnant in that it does not flow like a river or like the ocean.

However, the water is not completely still because there is a filter that is supposed to keep water circulation going.

It may look still on the surface, but if you have ever placed your feet on the floor of the pool you may have felt water sprouting out of the little pipes at the bottom.

This is the main job of the filter.

But if the filter is not working properly or if you have fitted a smaller filter for a much larger pool, then the circulation may not be enough to keep algae and other such contaminants out.

For example, even though a river has a flow you might notice algae in spots where the water is still or if it has been a particularly dry year and the river doesn’t have a strong enough current.

Similarly, if there are dead spots in the pool or if the water in one part of the pool is not being circulated properly, then that makes the pool more vulnerable to algae. It is advisable to do regular testing to ensure that the water is being circulated properly.

Insufficient Chlorine or Chemical Treatment

All swimming pools must have the appropriate amount of chlorine (or other sanitizers) in the water. In addition to maintaining the right level of chlorine in the water, you also need to ensure that the chemical has been evenly mixed in the water, which circles back to the point about proper circulation and filtering.

When chlorine interacts with water it creates hypochlorous acid. This is a weak acid that kills bacteria in the water like salmonella and E.coli. Proper chlorination will also arrest the development of algae spores.

There are various types of compounds you can use to chlorinate the water, including sodium hypochlorite and chlorinated isocyanurates.

If the chlorine levels in the pool are incredibly low or if it is not evenly distributed in the water, it gives a chance for algae growth to spread. You should also be aware of the effects of chlorine lock – have a read.

green algae in a swimming pool

Upset pH Balance

If you have a pool at home, it’s important to be sure that the pH levels in the water are being regularly tested. A pH test will tell you about the levels of alkalinity in the pool.

If the pH level is high, that means the alkalinity in the water is greater. While by itself that is not a bad thing, if higher alkalinity levels are combined with low chlorine then that could make your pool more vulnerable to algae.

In addition to checking the pH levels of your pool, you should also look for other determinants of water quality such as calcium and cyanuric acid levels.

It is important to have a proper balance of all the elements to keep the quality of your pool water up to par and prevent algae growth. Use a pool test kit regularly to check the quality.

dirty pool almost empty

Questions About Pool Algae

How Can I Prevent Algae Growth in My Swimming Pool?

You can prevent algae growth in your swimming pool by regularly cleaning and skimming the pool, keeping the water chemistry balanced, and minimizing sunlight exposure.

What Are the Different Types of Algae That Grow in Swimming Pools?

The most common type of algae found in swimming pools is green algae, though mustard algae, black algae, and pink bacteria can also be present.

What Is the Best Way to Treat and Remove Algae from a Swimming Pool?

To treat and remove algae from a swimming pool, you should use algaecide along with manual cleaning to get rid of as much of the visible growth as possible before shocking the pool with chlorine or other chemicals.

How Does Sunlight Affect Algae Growth in Swimming Pools?

Sunlight is essential for encouraging the growth of many types of pool algae, so it’s important to minimize its exposure by using a cover or shade when not in use.

How Often Should I Shock My Swimming Pool to Prevent Algae Growth?

To prevent the growth of algae in your swimming pool you should shock it once every week or two depending on usage levels and weather conditions.

Final Thoughts

Swimming pools require a fair amount of maintenance if you want to ensure a certain degree of sanitation is being followed. Algae buildup is one of the most common problems when the right conditions are present such as poor circulation, low chlorine levels, and an imbalance in pH levels.

The key to avoiding this issue is understanding why it happens so that you know how to prevent it from occurring in the first place by regularly testing the water quality and maintaining an appropriate level of chemicals in your pool.

Understanding the main causes of algae buildup in pools will help you to avoid all the possible causes at the early stages and keep your pool squeaky clean and fresh.

michael keenan author

Author - Michael Keenan

I'm Michael Keenan the owner and creator of the Outdoor Care Guide. I'm a trained horticulturist with over 30 years of experience in pool care, plant care, and landscape care! Seemed like a good idea to share - I think I can make your life easier and save you some time and money!