How to Get Rid of Mustard Algae in Your Pool

Written by Michael Dean
August 11, 2023

treating pool algae

Mustard algae will cause a yellowish cloud in your pool water that can get out of control fast. Although getting rid of mustard algae can be challenging, you should be able to kill it with the right tools and information.

Whether you want to prevent algae from forming or are struggling with a mustard algae outbreak, I’ll teach you everything you need to know.

Main Takeaways

  • Mustard algae may look like pollen or sand and is more commonly found in southern climates like California, Florida, and Texas.
  • Mustard algae is caused by heavy rain or after many people have used the pool.
  • To kill mustard algae, you must brush and triple shock the pool.
  • To prevent mustard algae from forming, keep your pool clean, maintain the pool chemistry, fun the filter, add algaecide and shock, and brush and vacuum the pool weekly.

So, What’s Mustard Algae?

Mustard algae (also called yellow algae) is like the cousin of green algae, but is more commonly seen in southern climates like California, Florida, and Texas. If you have mustard algae in your pool, it might look like a stain, pollen, some sand, or dirt. You’ll probably see it growing in areas that are typically shaded, like the sides or bottom of your swimming pool. Here’s a photo I found of mustard algae growing on the stairs and sides of the pool.

Although it’s not usually dangerous, too much mustard algae can house bacteria like E Coli.

What Causes Mustard Algae in the First Place?

Algae usually spills into the pool when heavy rain washes organic material into the water. It can also attach itself to swimwear and contaminate the water. It is common to have a mustard algae outbreak after many people have used the swimming pool.

Algae can bloom quickly depending on where you live and your particular situation. Things that affect the speed of an algae bloom include:

  • Warm, sunny days
  • High nitrates or carbon dioxide levels in your pool
  • Pool water that’s out of balance
  • Improper sanitation or filtration of pool water
  • Not enough circulation

How Do I Know If It’s Really Mustard Algae?

Mustard algae is not always easy to identify. It isn’t the slimy algae you may be used to, and because of this, it can often be mistaken for stains or dirt. It does not generally make the water very hazy, so if your pool is cloudy, read my guide on how to clear cloudy pool water.

Mustard algae is more likely to return than other types of algae because it is chlorine resistant. If you were to brush it away, it would likely grow back in the exact same place.

How to Kill Mustard Algae in Your Pool

Removing mustard algae is a difficult task. However, by taking these simple steps, you can get everything looking crystal clear again in no time.

1. Remove Everything From Your Pool and Clean It

The first step is to remove everything from your pool. That means any equipment, floats, and toys. Anything in the water could contain algae, so disinfect it with a solution containing chlorine (but not bleach). You’ll also want to thoroughly wash all your bathing suits and other swimwear.

2. Brush and Vacuum the Mustard Algae

Brush the walls of your pool to remove any algae hanging there. Let everything settle before using a manual pool vacuum. Ensure your filter is set to waste mode so the vacuumed water doesn’t release back into the pool. And finish off by topping your pool with fresh water to replace any water that was vacuumed.

3. Balance The Chemicals In Your Pool Water

Balancing the pool water is an essential step after adding more water. Clean and inspect your pool’s filtration system. Then ensure the pH levels are correct. Once you’ve done that, make sure your pH levels are correct. Using a pool test kit, ensure your alkalinity level is 120-150 parts per million and your pH level is 7.4-7.6. Algae love to bloom in water with higher pH levels, so keeping the balance is essential.

Poolmaster 5-Way Test Kit

This is the best liquid testing kit on the market based on my testing.

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4. Brush Again

Before you shock your pool water, brush your pool walls and floor again. If you have a vinyl pool, use nylon brushes; use steel bristle brushes if you have a plaster pool. Add some algaecide — which you can find online or at a hardware store — and brush your pool one more time. I know it may sound repetitive, but getting rid of this stubborn algae is necessary.

LALAPOOL Swimming Pool Wall & Tile Brush

Cheap, easy to use, and the bristles hold up pretty well to repeated use.

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5. Shock Your Pool Water

Now it’s time to shock your pool. Follow the instructions on the shock you purchase and multiply everything by 3 to ensure you eliminate all the mustard algae. I recommend using 3 pounds of shock for every 10,000 gallons of pool water. If you don’t know already, you can use my pool volume calculator to figure out how many approximate gallons of water are in your pool. You can also read my article on how to shock your pool for all the steps. And remember to shock your pool at dusk, because sunlight can lessen the impact of the chemicals.

In The Swim Calcium Hypochlorite Chlorine Granular Pool Shock

In The Swim has a reliable cal hypo shock that is effective and easy to use.

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6. Circulate That Water!

Use the automatic cleaning settings to circulate your swimming pool. You should also run the filter for at least 24 hours.

7. Keep Brushing and Adjusting Chlorine Levels

Over the next three days or so, continue brushing your pool walls to encourage the chlorine shock to do its job in killing the algae. Additionally, keep testing and adjusting your levels to make sure the chlorine stays highly effective.

8. Ensure the Mustard Algae is Gone

Only put floats and toys back into the swimming pool when the algae is completely gone. Mustard algae is very stubborn, so the minute you notice it, start treating it. If the first shock doesn’t work, keep shocking your pool. Note that it will latch onto anything that is in your pool. Sanitize items well so that algae is not reintroduced.

Get My Free Pool Care Checklist

Download my free, printable pool maintenance checklist to help you accomplish regular pool care tasks for any type of swimming pool.

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How to Prevent Mustard Algae From Coming Back and Taking Over Your Pool

Unfortunately, it’s not uncommon for mustard algae to come back.

However, you can do a few things to stop the mustard algae from taking over your swimming pool.

Regular use can help prevent algae from forming because it is harder for the algae to stick to the walls and spread when the water moves around. On the other hand, algae commonly enters the swimming pool from swimwear, so be sure to keep your swimming trunks clean!

Maintain the proper levels of pH, alkalinity, chlorine, etc.

Keeping your pool clean is critical in preventing algae growth. That includes cleaning equipment and items like pool ladders and toys.

Run the filter for about 8-12 hours daily.

Brush and vacuum your pool’s surfaces weekly. You can invest in a robotic pool cleaner to help lighten the load (check out my research on the best robotic pool cleaner for recommendations).

Add algaecide and shock to your swimming pool every week to help prevent mustard algae.

Algae can piggyback on anything: poles, diving boards, swimmer baskets, pool toys, nets, slides, covers, brushes, steps, ladders, floats, you name it. Use a cleaner with chlorine to disinfect your pool equipment regularly.

If you follow all of the above methods and cannot successfully get rid of the algae, you will need to drain the pool. Use acid or pressure wash the walls to kill any algae roots that could be embedded in your pool. After this, clean or change out the filter.

Mustard algae can be incredibly irritating and extremely difficult to remove from your swimming pool. However, it should not cause panic, as there are ways to kill this contaminant and make your pool sparkle once again.

Still have more questions about mustard algae? Let me know.

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