How can I tell if I Have Mustard Algae in my Pool?


The proper question is… what is mustard algae? If you ever spot a “dirt” sticking to your pool walls or there’s something that appears like a “pool stain” from a distance, grab your brush and chlorine. Do the following tricks to know if it’s mustard algae planning to make your pool yellow. 

First, observe the water around the “dirt.” If it isn’t cloudy, brush the spot. Did it get muddy after a few strokes of brush around this supposed “dirt?” If your answer is yes, you’re dealing with mustard algae.

Second, go near the pool stain to confirm if it has a dry, powdery surface. Sprinkle granular chlorine and see if it fades. Did it disappear when you scatter chlorine over it? If yes, these are mustard algae conniving to surprise you with yellow pool.

Lastly, if the dirt floats and is slimy, you’re dealing with another algal problem caused by black or green algae. Yellow mustard algae thrive in shaded wetland areas like the bottom or sides of your pool.

What causes mustard algae?

Unbalanced pool water chemistry allows the development of yellow mustard algae, striving to make your pool yellow. Their growth usually happens due to filter and pump problems or environmental factors. With that, you can also say that a yellow pool can diagnose a pool filtration issue.

Running a pool filtration system only for a few hours every day is not enough to clean an entire pool. While pool filtration rate should depend on your pool size and additional equipment and features you have, it should take 8 hours a day on average for the pool pump to circulate and clean your water.

Constantly exposing your pool to heavy rains combined with sunlight ruins your pool chemistry. Why? Heavy rain water dilutes the chlorine you added to your pool water, brings debris, and adds unwanted microorganisms to your water. Also, the sunlight causes evaporation of your pool water and chlorine simultaneously. All of these, hand-in-hand, design the perfect breeding environment of yellow mustard algae.

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How to treat mustard algae?

There are two ways to treat your yellow pool. Manual cleaning requires more effort and time than simply using chemicals. However, it’s highly recommended to apply both of these techniques to ensure there are no yellow mustard algae lurking around and that they’re never coming back.

Clean manually:

  • Laundry your bathing suits and wipe down your pool toys, ladders, wall fittings, swimming pool gears with a chlorine-based cleaner because these could be where yellow mustard algae are possibly clinging to life.
  • Make sure your pool filter is clean because you don’t want a dirty filter to add algae spores back into your yellow pool.
  • Do a good brushing and vacuuming around the pool area before shocking your pool. Ensure you see no spots and stains wherever you set your eyes on.
  • Test the balance of your pool water. If not satisfied, don’t get tired of doing these things repeatedly. Stop only when you get the right pool water chemistry. 

Use chemicals:

  • Shock your pool with the right amount of chlorine level (general use is ten ppm and 30 ppm for 8 to 24 hours). Remember that lower chlorine levels will render ineffective disinfection while higher levels will cause red eyes and itchy skin.
  • Choose a pool and spa algaecide that contains 60% polyquaternium WSCP and consider a non-foaming algae killer if you don’t want your pool to foam. 
  • Avoid shocking and adding algaecide in your pool together. You should first shock and wait for the chlorine levels to subside below 5 PPM because mixing high levels of chlorine and algaecide makes both of them useless. 

Is mustard algae dangerous?

Yes! Regardless of type and color, algae are dangerous without exemptions. Accidentally ingesting water with bacteria and algae can cause fever, diarrhea, and other symptoms associated with infection. Also, when you’re exposed to yellow mustard algae, you can have skin rashes, and bacteria can break into the skin to bring you more severe infections later on.

Algae poisoning only gets treated in the hospital. Know that there are no home remedies once you get infected with yellow algae mustard. The only thing you can do is make sure no one in your family is exposed to a yellow pool.

I get you. Being a pool owner entails strict compliance and adherence to laws for the safety and maintenance of your pool. But pool cleaning and sanitizing should not be considered problematic and complex tasks if you are doing them correctly. 

After all, you bought yourself a swimming pool to improve your health and bring fun to you and your children. So treat your pool right, and it will treat you right back. 


  1. What is mustard algae? Yellow mustard algae look like a “dirt” or “stain,” clinging to solid materials in shaded wetland areas like the bottom or sides of your pool.
  2. What causes mustard algae? Unbalanced pool water chemistry. 
  3. Is mustard algae dangerous? Yes
  4. How to treat mustard algae? Shock your pool and use algaecide after to treat your yellow pool but don’t put them simultaneously. 

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