Swimming Pool Guides & Reviews by Certified Pool & Spa Operator®

How to Remove Mustard Algae from Your Pool

Here’s Everything You Need To Know

Mustard algae might sound like something you put on a hot dog, but it is actually a common type of algae found in swimming pools. It is sometimes called yellow algae because of its yellow color. If you don’t maintain the cleanliness of your pool, then don’t be surprised if you start seeing this algae form.
Although mustard algae are rather rare, it can still show up in certain circumstances. Fortunately, it is easy to eliminate mustard algae and keep it away by following a couple of simple steps.

Overview of Mustard Algae

You probably already know about slimy green algae and how it sticks to pool walls and forms flat blobs in the water. Mustard algae are a little different, though. People tend to confuse mustard algae with pool stains, sand or dirt. The algae belong to the “xanthophytes” family of microbes. They’re resistant to chlorine, so you can’t get rid of them with standard sanitizing chemicals.
Mustard algae will stick onto any surfaces or items in your pool, such as the walls. But it can’t live anywhere else but in your pool. You’ll find the algae on your pool toys, equipment, floats, and sometimes on the bathing suits of the swimmers. If you notice any mustard algae around, apply disinfectant to the algae prone areas right away.

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The 10 Steps for Killing Mustard Algae

Don’t let mustard algae keep you down and depressed. Here are ten easy steps to kill mustard algae in order to keep your pool sanitized.

1) Wash Bathing Suits in a Washing Machine

If you have mustard algae on your bathing suits or clothes, then put your messy laundry in the washing machine. The algae love natural fabrics, so you must clean them off aggressively. Use bleach too.

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2) Clean the Toys and Accessories

Clean all the accessories and toys of your pool with a chlorine cleaning solution and a clean, soft cloth. Just apply some of the solution to the cloth and wipe down the toys and accessories with it.
Don’t use bleach because some of the rubber and plastic materials of these items could get damaged by it. A multipurpose chlorine-based cleaner is better for eliminating mustard algae from these items without causing damage to the materials.
If bleach is the only thing you have available, then it must be diluted in water first. Use 1-part bleach, ten parts water. Apply to the necessary surfaces.

3) Relocate Pool Equipment to Shallow Area of the Pool

Relocate all the items in your pool and put them at the shallow end. This includes your poles, hoses, and any other maintenance items that were affected by the mustard algae. If something bulky has mustard algae on it and you can’t clean it manually, then put it in the pool too with the other items.
You’ll want to keep all the infected items together in the pool at the shallow end. That way, you can disinfect all the items easily after you put the sanitizing solution in the pool.

4) Vacuuming and Brushing

Before you start sanitizing your pool water again, it helps to get rid of the existing mustard algae first. You can do this with a high-quality algae brush which is designed to loosen all the algae, so they’re not so clingy. Once you do this, you can easily suck up the algae with a manual vacuum. Since mustard algae may inhabit your filtration system, turn the filter’s setting on “waste” instead of “backwash” prior to turning on the vacuum.
After you’ve vacuumed the algae out of the pool, put the garden hose in your pool and add more freshwater to it. The vacuum sucked up water before, so you’re just replenishing what was lost. .

5) Test the Water and Balance If Needed

Algae spread when pool water doesn’t have enough sanitizing solution, or its water chemistry is imbalanced. You must test the pool water with test strips to determine its pH level and alkalinity level. 
Based on the test results, add the proper balancing chemicals to the water if necessary. You’ll want the alkalinity to be between 100 ppm (parts per million) and 150 ppm. As for the pH level, that should be between 7.4 and 7.6. If you can keep the water chemistry balanced, then shocking the pool will have a better effect. The mustard algae won’t stand a chance at survival.

6) Brush Again

Use the same brush from before and scrub down the pool again. Any existing algae that is left will get broken up some more. If you see the algae floating in the pool water, then use a shock treatment to kill it.

7) Triple the Shock Treatment

One pool shock treatment won’t be enough to get rid of mustard algae in the water. Therefore, it is recommended that you use 3 lbs. of shock to clean 10,000 gallons of water. Do it when the sun goes down. Turn on your pump and filtration system. Leave them on for the next 24 hours. The algae should be gone by then.

8) Brush and Balance Again

Brush your pool walls again to ensure that no mustard algae is clinging to them. Test your chlorine levels and add more if needed. Also, test your level of alkalinity and pH regularly and make the proper adjustments to them too.

9) One More Shock

Wait a couple of days and give your pool one more shock treatment. You can use a lower strength of shock this time, like 1 lb. per 10,000 gallons of water. This will eliminate the few remaining algae that may still be in the water.

10) Test the Water to Make Sure There is No Algae

Do one more water chemistry test to ensure there are no algae. If the reading shows the water is balanced and everything looks clear in the water, then your mustard algae problem has been resolved.
Any pool equipment or accessories that you put in the pool during this process can be removed.


Now the only thing left to worry about is preventing the mustard algae from returning again. If you keep your water chemistry balanced, then everything should be fine. This means balancing the sanitizer, pH, and alkalinity levels regularly.
Leave the filtration system and pump running for between 8 and 12 hours per day throughout the entire season. Brush and vacuum your pool if you notice any signs of algae.
Shock treatments should be made once per week. Use 1 lb. of shock per 10,000 gallons of water. Clean all the pool items, equipment, and accessories, such as solar blankets, slides, diving boards, steps, ladders, floats, and toys. Use the cleaning solution with chlorine in it to do this. This will ensure the algae is killed.


The only mustard you need to worry about now is the kind that goes on your hot dogs and hamburgers. All other mustard, especially in your pool, has got to go.

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