What Makes A Pool Turn Green?

A well-maintained pool usually has a sparkling blue color and smells fresh. But sometimes, pools turn green, making them look pretty uninviting. So, what causes this green color to appear?

Your pool has probably turned green due to algae growth from low chlorine. Your pool might also turn green if its pH levels are high, or if the alkalinity is too low (which are things that basically lead to insufficient chlorine). Heavy rain or partial refilling can cause these imbalances.

The truth is, a green pool is as uninviting as it is a health risk. In this guide, we’ll look at the causes of a green pool in detail and consider whether you can still swim in the green water. We’ll also take you through a step-by-step guide to cleaning your pool and restoring its normal color. Let’s get started!

What Makes a Pool Turn Green?

Green swimming pool with teak wood flooring

If your pool is no longer an appealing shade of blue, the first step should be to pinpoint the cause of the green water. Here are some common reasons why pools change color:

Algae Growth

Algae is often the root of many swimming pool problems and could be why your water is turning green. Although it isn’t dangerous or toxic, algae can lead to other health hazards if not treated properly.

Algae growth turns a pool green because it contains chlorophyll, which absorbs light energy from the sun. It then uses this energy to convert carbon dioxide and water into oxygen and carbohydrates through photosynthesis.

These microorganisms also grow when your pool lacks enough chlorine, since the chemical is key to preventing algae growth.

High pH

The best pH for pools is between 7.4 and 7.6. If it rises above this level, your pool will become a suitable habitat for microorganisms. Additionally, at high pH levels, chlorine becomes inactive and ineffective, making it more easy for algae to grow.

Make sure you keep an eye on your pool’s pH by having it tested or testing it yourself with a chemical test kit (on Amazon).

Inadequate Filtration

One of the best ways to keep your pool clean and inviting is proper filtration. It’s recommended to run your filter at least three times a day. This will ensure that at least 90% of the water is filtered and most of the organic matter removed.

High Levels of Phosphates

Phosphates are mainly found in fertilizers and may find their way into your pool through runoff and detergents. And because they’re one of the best algae foods, fertilizers can exponentially increase the algae population.

Additionally, high phosphate concentration inhibits chlorine action. So make sure it’s below 500 parts per billion if you want to maintain your pool’s characteristic blue color.

High Levels of Cyanuric Acid (CYA)

CYA is an essential component of pool treatments because it helps chlorine last longer. It’s a pool stabilizer, and you can’t do without it. However, too much of it is harmful to your pool and impacts the effectiveness of chlorine.

If it exceeds its recommended level of 30-50 parts per million, it makes chlorine unstable and thus degrades in sunlight.  

Algae in Swimming Pools

Algae can grow in most bodies of water, and swimming pools are no exception. Even though there are various types of algae, the most common type in pools is hair algae or cyanobacteria.

It’s often referred to as “green slime” because it looks like stringy green hair growing out of your swimming pool walls and bottom. Green slime sticks on your pool’s walls because of the light, which offers a conducive environment for growth.

There are many types of algae, but there are four major types based on color classification:

Pink algae

Pink algae is also known as “water murk.” It often appears pinkish-red or brownish-red in color, depending on how much sunlight it receives. Pink algae thrive in warm waters when there are high levels of phosphorus present.

Pink algae doesn’t spread as fast as its green counterpart but is still problematic. The best way to prevent this type of growth from occurring is by keeping your pool water well balanced so that there aren’t any excess nutrients for this type of bacteria to feed on.

Black Algae

This type of algae is one of the most problematic, especially if left to spread. It’s a dark-colored, thick growth that can clog up the filters of your pool or hot tub. If you have black algae in your pool, getting rid of it as soon as possible is crucial.

The main reason you may have black algae in your pools is if it was introduced from outside sources like the ocean. It also grows more when there is poor circulation of water throughout the pool or if there isn’t enough sunlight reaching the water’s surface area.  

Yellow Algae

Yellow algae is often referred to as mustard algae because it looks like mustard and often has a butterscotch color. The yellow color can range from bright yellow to dark brown, depending on the type of bacteria added to the water.

Yellow algae can be found anywhere in the pool, including walls and floors, but it typically starts growing on the pool’s sides where no chemicals are present.

This algae grows easily because it needs minimal light, oxygen, and nutrients (food). It thrives in low-light areas because it has chlorophyll inside its body, which allows it to convert sunlight into energy through photosynthesis.

This process enables yellow algae to survive even when there aren’t enough nutrients in the pool water.

Green Algae

Green algae is bright green in color and grows on the walls and floors of your swimming pool. It’s one of the first types of algae that you’ll see in your pool, and it grows more quickly than others.

This type of algae usually appears in the spring or summer months when there’s more sunlight for it to grow. The best way to control it is by adding chlorine to your pool water. You should, therefore, test your chlorine levels daily to determine how much chlorine is needed to keep the green algae from growing.

Can You Swim in a Pool With Algae?

Although you can swim in a pool with algae, it’s not recommended because there’s the possibility of certain medical conditions such as:

How to Clean a Green Pool

A pool turns green when there is algae in the water

It’s important to note that there is no one-size-fits-all solution to cleaning green pools. However, following these steps will provide you with exemplary results.

  1. Assess the pool: Assessing the pool will help you determine if you need to drain it or not. If the algae growth is widespread, draining is recommended. However, if not, a simple clean-up will suffice.
  2. Take tests: Test the PH and chlorine levels to help decide the best action. You should also test the sanitizer levels to ensure they are within their appropriate parameters.
  3. Clean and shock the pool: The next step is cleaning the pool (if you didn’t drain it). Concentrate on areas with low circulation and dirt. You should also clean and empty the skimmer baskets. After this, apply chlorine in high concentrations, about 30 ppm.
  4. Use an algaecide and then filter: Apply an algaecide to kill algae and let the filter do its work by running it for at least 24 hours. This will remove any residue and make your pool ‘brand new.’

Pool Maintenance Tips

One of the best ways to prevent your pool from turning green is with regular maintenance. Some of the best maintenance tips are as follows:

  • Keep your pool clean. If you have a natural swimming area, vacuum the water and remove any leaves or other debris as needed.
  • Keep your chemicals balanced. Test your water regularly and make sure you’re doing everything right to keep it clean and clear.
  • Keep your filter clean. A dirty filter can cause brown algae growth on the walls of a pool if it’s not maintained properly, so keep yours running smoothly by using a cleaner designed specifically for this purpose.

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