Mustard Algae in Pool: Causes, Prevention, and Treatment

By Algal Web

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If you own a pool, you’re probably well aware of the challenges that come with keeping it clean and clear. One of the most frustrating issues that pool owners can face is the growth of mustard algae.

This type of algae is notorious for its yellow or brownish color and its resistance to normal chlorine levels.

In this article, we’ll explore the causes of mustard algae growth, how to identify it in your pool, and what you can do to prevent and treat it.

Before we dive into the details, let’s take a moment to define what we mean by mustard algae. This type of algae is a chlorophyte that thrives in warm, sunlit areas of your pool.

It’s often mistaken for sand, dirt, or pollen, and can be hard to remove once it takes hold. Mustard algae can be introduced to your pool by swimmers, pool toys, or even the wind, and once it’s established, it can quickly spread.

Causes of Mustard Algae Growth

Understanding the causes of mustard algae growth is key to preventing it from taking over your pool. Some of the most common causes include:

Inadequate Sanitation:

Sanitation is one of the most important factors in preventing mustard algae growth. If your pool’s chlorine levels are too low, it can create the perfect environment for mustard algae to grow.

Chlorine is an essential chemical used in pool maintenance that helps to kill bacteria and algae and prevent their growth.

If your chlorine levels are not maintained, it can lead to the growth of mustard algae in your pool. To prevent this, make sure to regularly test and balance your pool’s chemical levels.

Warm Water Temperatures: 

Mustard algae thrive in warm water temperatures, which is why it’s most common in the summer months. In warmer water, algae can grow more quickly, leading to an increase in algae blooms.

To prevent this, it’s important to keep your pool water at a lower temperature. You can do this by using a pool cover or by running your pool’s filtration system during the hottest parts of the day.

Poor Water Circulation: 

If your pool’s water is not circulating properly, it can create stagnant areas where algae can grow. This can be caused by a clogged filter, a broken pump, or other issues with your pool’s circulation system.

To prevent poor water circulation, make sure to regularly clean and maintain your pool’s filter and pump, and ensure that your pool’s circulation system is working correctly.

pH Imbalances: 

If your pool’s pH levels are too high or too low, it can create an environment that’s conducive to algae growth. If your pH is too high, it can lead to cloudy water and scale buildup, making it easier for algae to grow.

If your pH is too low, it can cause skin and eye irritation and corrode pool equipment. To maintain the proper pH level in your pool, make sure to test and balance your pool’s pH regularly.

Use a pH increaser or decreaser as necessary to bring the levels back to the recommended range.

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How to Identify Mustard Algae in Your Pool

The first step in treating mustard algae is identifying it. Here are some of the most common signs of mustard algae growth:

Yellow or Brownish Stains:

One of the most common signs of mustard algae growth is yellow or brownish stains on your pool’s walls or floor. These stains may look like dirt or sand, but they won’t brush away easily.

You may notice these stains in the corners of your pool, near ladders, or in areas with poor circulation.

Brush Resistance:

Mustard algae are resistant to normal brushing and vacuuming. Unlike other types of algae, it can attach itself firmly to pool surfaces, making it difficult to remove.

If you notice that your pool’s stains are not coming off with normal brushing, it may be a sign of mustard algae growth.

Cloudy Water:

Mustard algae can make your pool water appear cloudy or hazy. This is because the algae clump together and create a film on the surface of the water.

If you notice that your pool water looks cloudy or hazy, it may be a sign that mustard algae are present.

Musty Odor:

If your pool smells musty or stale, it may be a sign that mustard algae are present. This odor is caused by the algae breaking down and releasing organic compounds into the water.

If you notice a musty smell in your pool area, it’s important to address it immediately to prevent the growth and spread of mustard algae.

Prevention Tips to Keep Mustard Algae at Bay

Prevention is always the best strategy when it comes to pool maintenance. Here are some tips to help you keep mustard algae from taking hold in your pool:

Maintain Proper Sanitation:

To keep mustard algae at bay, it’s important to maintain proper sanitation in your pool. This means keeping your pool’s chlorine levels within the recommended range for your pool size and usage.

It’s also important to test your water regularly to make sure the levels are where they need to be.

Shock Your Pool Regularly:

In addition to maintaining proper sanitation, it’s a good idea to shock your pool regularly. Shock treatments can help to kill any algae spores that may be present in your pool.

This is especially important after heavy pool usage, rainstorms, or any other event that may introduce new contaminants to the water.

Brush and Vacuum Your Pool:

Regular brushing and vacuuming can help to remove any algae spores before they have a chance to take hold.

It’s a good idea to brush your pool walls and floor at least once a week and to vacuum your pool regularly to keep it free of debris.

Keep Your Water Circulating:

Stagnant water can create the perfect environment for algae to grow, which is why it’s important to keep your water circulating.

Use your pool’s pump and filter to keep the water moving, and make sure to clean and replace your filter regularly to keep it working properly.

Balance Your pH Levels:

Maintaining the proper pH balance in your pool is another important step in preventing mustard algae growth.

Test your pool’s pH levels regularly and make adjustments as needed to keep them within the recommended range. A pH level that is too high or too low can create an environment that’s conducive to algae growth.

Effective Treatment Options for Mustard Algae Infestations

If you do find mustard algae in your pool, don’t despair. There are several effective treatment options that can help to eliminate it:

  • Algaecides: Algaecides are chemicals that are designed to kill algae. Look for an algaecide that’s specifically formulated for mustard algae.
  • Brushing: Use a stiff brush to scrub the affected areas of your pool. This will help to break up the algae and make it easier for the algaecide to work.
  • Shock Treatments: Shock treatments involve adding a large dose of chlorine to your pool to kill off any algae spores. Make sure to follow the manufacturer’s instructions carefully.
  • Vacuuming: After you’ve treated your pool with an algaecide and a shock treatment, you’ll need to vacuum up any dead algae that have settled on the floor of your pool.

Maintaining Your Pool After Treating Mustard Algae

Once you’ve successfully eliminated mustard algae from your pool, it’s important to take steps to keep it from coming back. Here are some tips to help you maintain a healthy, algae-free pool:

  • Test Your Water: Test your pool’s water regularly to make sure that the pH and chlorine levels are within the recommended range.
  • Clean Your Pool Regularly: Keep your pool clean by brushing and vacuuming it regularly. This will help to remove any debris or algae spores that may be present.
  • Use Algaecides Preventatively: Consider using an algaecide on a regular basis to prevent the growth of mustard algae.
  • Balance Your Chemicals: Make sure to keep your pool’s chemicals in balance to prevent algae growth. This includes pH, alkalinity, and calcium hardness levels.


Mustard algae can be a frustrating problem for pool owners, but with the right prevention and treatment strategies, you can keep your pool clean and clear all season long.

By maintaining proper sanitation, keeping your water circulating, and using the right chemicals, you can prevent the growth of mustard algae and other types of algae in your pool.

If you do find mustard algae in your pool, don’t panic. With the right treatment options and regular maintenance, you can get rid of it and enjoy a sparkling, algae-free pool all season long.

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