Mustard Algae or Sand – Here’s How To Know ( With Picture)

Do you have a swimming pool in your backyard, if yes, then at some point of time you will be confused whether this sticky substance is mustard algae or sand? . But let me tell you that this is mustard algae. 

Mustard algae or sand is the enemy of your swimming pool, it is mostly found on the floor of your swimming pool where sunlight does not come directly. The biggest damage caused by mustard algae is the low chlorine level in your swimming pool, which can drop by 1 ppm in one night.

When the pool is covered for a long time, due to poor circulation of water and lack of sunlight, mustard algae flourish, many times people get confused and consider it as sand, pollen or dust.

In this article, I will tell you the difference between mustard algae and sand, how it looks with some example pictures of mustard algae in the pool, how to test for mustard algae and how you can save your pool from mustard algae and sand, so definitely read this article till the end.

What is Mustard Algae ?

Mustard algae is one of the members of the Algae kingdom. in a scientific term known as “Pseudomonas aeruginosa” or “Yellow Algae”. It is a type of algae that commonly grows in swimming pools and other water systems. It gets its name from its yellow or mustard-like color. Mustard algae can be a nuisance in pools as it can create slimy and slippery surfaces, making the pool less enjoyable and potentially unsafe for swimmers.

Mustard algae is rare but it happens and it doesn’t really look like algae. Sometimes it can look like pollen or it can look like sand. It likes to stick to things, not just on the pool floor . it might be stick on your pool floats or your pool equipment. Most of the people think it grows in the but but its not fully true sometimes it comes from outside the pool. The biggest problem it’s that it is chlorine resistant .

types of algae
types of Algae in pool –

Characteristics of Sand and Pollen

Sand is the largest particle of soil approx 0.6 – 2.0 mm. It feels like a rough surface in your pool. So the question is “why is there sand in your pool?” well might be the culprit is your pool sand filter. Yes it happens when pool filters aren’t able to filter you must check once a week that sand filters work or not.

Where algae clings to the walls of the pool, the pollen floats on top of the pool and is sometimes cleared by the pool filter system. It mostly comes from trees and plants in your backyard. People sometimes get confused by algae and pollen.

How to Differentiate Between Mustard Algae and Sand?

Quick disclaimer everyone has a different way to test for mustard algae in the pool and how to get rid of yellow mustard algae. I am sharing my own way of doing based on all of the years in the pool industry and the research that I have done.

Here is the few step that help you to differentiate Between Algae and Sand:

The brush test

This is the most simplest test you can perform to identify between mustard algae or sand. But it takes time to show results. In some cases it took 6 hours but most of the time 12 to 24 hours is enough to get the result. You just need to clean the entire pool, especially plaster or Pebble finish surface with a brush.

You can use this brush to give the best cleaning result. I highly recommend my team members a dedicated work boot like this boot that helps to protect your food from algae infection.

After cleaning the pool wait for 12 to 24 hours if the patches reappear it means you do have mustard algae.

You should notice that while cleaning the pool you feel trouble in brushing off the dirt. It happens when your pool has sand . sand doesn’t come off easily with brush; it usually requires an agent like detergent. On the other hand mustard algae can be brushed off easily.

Check Condition Of swimming Pool

With a year of experience we observed that mustard algae never accumulate in the bottom of the pool .it always sticks with the pool wall. If we talk about sand it is always found in the form of a layer at the bottom and pool wall.

Texture Test

This method is one of the easiest methods you can do by yourself . Always remember that mustard algae have a slimy and smooth texture that can easily stick with any surface. On the other hand, sand has a rough texture and can’t stick with any surface.

Always observe your Poll wall

Mustard algae initially take some time to grow, but once it completely grows it can escalate rapidly. In the beginning I mentioned that mustard algae belongs to the algae family. Algae are living organisms that use oxygen & sunlight for their growth. It can easily climb the pool wall whereas dirt and calcium scales don’t.

Jar Or Bucket Test

Even after this whole test, if you are unable to differentiate between mustard algae and sand, then you should try the jar and bucket test. For this, you have to take a transparent jar or bucket, in that you take some samples of your pool and keep it in a shady place, and leave it for about 2-3 days. If you see more patches than before in the jar or bucket, then it is mustard algae only.

Check Chlorine Level

The chlorine test is a sure and reliable way to check if you have mustard algae in your pool. Overnight Chlorine Loss Test (OCLT) This chlorine test is done overnight after the sun has gone down.

How does OCLT work?

OCLT works because chlorine can be removed from the pool in only two ways: through sunlight or through living organic matter such as mustard algae. This is the main reason for doing the OCLT after sunset.

  • To prepare for the exam, you must ensure that you have the right test kit with you. If I am allowed, I will give you the Fas-DPD test done. This is a simple test where you get the result of both chlorine free and combined chlorine by Titration and drop count test.
  • Take the first reading after the sun sets and write it in a notebook, and take the reading before the sun rises in the morning. If there is a drop of 1 ppm in the chlorine test reading, then you have mustard algae in your pool.
  • Before vaulting you need to take care of some things. 
  • Do not add chlorine to the pool before or during the test. If you already have a chlorine tablet inserted, remove it and wait 30 minutes.
  • Do a chlorine test in the morning before sunrise.

How To Get Rid of Mustard Algae?

For the mustard algae treatment Pool shocking is the most and easiest way to clean mustard algae, but with this you can follow some other steps as well, which will increase the chance of mustard algae not coming again.

Step 1: Wash your Swimwear and Pool Equipment’s and accessories

The first thing you should do is clean your swimming suits and equipment. Mustard algae can enter your pool from outside sources, such as swimwear, so you need to clean it thoroughly.

It would be better to use color safe bleach for clothes, this will give you more disinfection and the colors of swimwear will also be safe.

After this, you have to clean your swimming equipment and accessories properly, for this you can use bleach based disinfectant like Clorix, this is a better spray to clean your equipment. If you can’t find Clorex, you can use household bleach. You can mix 10 parts of water to one part of Bleach and spray all your equipment with it.

Step 2: Brush and vacuum your pool

Now you have to clean the algae sticking to the walls of your pool, this will make the next step of pool shocking very effective.

You must pay attention to which type of brush you want to use in the pool. If your pool is made of concrete, then you have to use a metal brush so that the algae can be cleaned properly.

If the pool is vinyl or fiberglass, a thick bristle plastic brush should be used.

Try to clean as much mustard algae as possible with a brush, for the rest you can use vacuum, but make sure that your vacuum is manual and set it to filter to waste mode. This will ensure that all the algae is removed from your pool if you vacuum in the circulate mode, the algae will not be removed from the pool.

Step 3: Test and balance the pH Level of Water

After this step, we are going to add too much chemical in the pool, so before that we have to make sure that the pH level of our pool is 7.4 to 7.6 or alkalinity is 100-150 ppm, so that the pool shocking step is very effective. yes. If the pH level of your pool is not correct then you can use ph level increaser or decreaser just keep in mind that the pH level of the pool is balanced before the pool shocking step

Step 4: Triple Shock the pool with Calcium Hypochlorite

We Gonna use Calcium Hypochlorite for this process,this is one of the best mustard algae killer. remember that this will increase the calcium hardness of your pool. If you do not want to increase calcium hardness, you can also use dichlor or trichlor. When your pool’s pH level is balanced, add 3lbs Calcium Hypochlorite Shock per 10,000 gallons of pool water and turn on the pool pump for 24 hours to allow the shock to dissolve completely. Do this step after the sun has set so that the chlorine is as strong as possible at that time.

Step 5: Keep Chlorine high and brush your pool Again

After 24 hours of being shocked, you will have to brush the pool again and clean it properly so that the algae are completely removed.

Along with this, you have to keep the pH level balance of the pool so that the chlorine remains as effective as possible.

After all these steps, if you want to be completely sure, then you can give one more shock and leave it at night, so that mustard algae can be completely gone.

I wrote a detailed article about how long it takes to clean . i think you must check this out after this article.

How To Get Rid of Mustard Algae without Any Chemical?

I found this method on youtube and where a person shared his experience cleaning algae . He uses t-shirts as a brush and cleans the whole pool like a brush method . I embedded his youtube video in this article. You can try this. The reason why I am sharing his video is because people tried this method and they got positive results. Most of the comments are positive. So you must watch the video.

But remember the only effective way to get rid of mustard algae is chlorine shock. If you don’t want to waste your time, just go with a shocking method.

How to Prevent Mustard Algae from Coming Back?

There are many ways to keep your pool free from mustard algae or sand but I am sharing with you my personal method that I use everyday.

  • Keeping pH Level Balanced: always make sure to maintain a balanced pH level and monitor Chlorine level.
  • Water circulation: Water Circulation makes the pool environment clean and resist to grow mustard algae or any kind of algae. Make sure that all pool filters and pump run 8-10 hours a day.
  • Shock on a Weekly base: We understand that not everyone has much time to spend on cleaning the pool, so you can prevent algae from coming to the pool by giving a small shock once a week, using 1 lbs of shock in 10000 gallons of water.
  • Wash Your Pool accessories regularly: Always make sure that your swimwear or pool equipment is thoroughly cleaned, for this you can use bleach or disinfectant.
  • Add phosphate remover: Mustard algae food is phosphate. Add some Onance Phosphate Remover to your pool to eliminate the source of food for the algae.


Is it mustard algae or sand in my pool?

If the chlorine level of your pool is low and there is a distinct slimy slimy feel in the shady area of ​​the pool, then there is a high chance that it is mustard algae. There are different types of methods to find out the difference between mustard algae and sand.

How can you tell the difference between sand and mustard algae?

You can find out by doing some types of tests in your pool such as:
The brush test , texture test and Jar Test

Will the shocking pool get rid of mustard algae?

Yes, you can kill mustard algae in a few days with proper steps and a solution of triple calcium hypochlorite shock.

How long does it take to clear up mustard algae in the pool?

 It takes a maximum of 2 days after Completing the shock method.

Can I swim in a pool with mustard algae?

We don’t recommend it because red algae can make you sick and mustard algae belongs to the algae family.

Does rain cause mustard algae in pool?

Mustard algae always come in your pool from outside, so rain and wind can also introduce mustard algae into your pool.

Does mustard algae eat chlorine?

Mustard algae is a chlorine resist . they used to eat phosphate in your pool.

Does Clorox algaecide work on mustard algae?

Yes, With the help of Clorox algaecide you can disinfect your pool equipment.

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