How to Get Rid of Mustard Algae in Your Pool—The Right Way

Would you like a side of algae with that pool party barbeque? Didn’t think so. Mustard algae might be considerably less slimy and visible than its green cousin, but it’s actually much more persistent in (and even out!) of your water.

If you’ve discovered mustard algae in your pool, it’s time to act fast before its spores spread out and it becomes even more difficult to eradicate from your pool. I’ll explain what exactly mustard or yellow algae is, where it comes from, how to test it against its lookalike (that’s sand), and how to get it out of your pool.

Getting rid of mustard algae in your pool means more (and safer!) poolside barbeques. Thankfully when it comes to grilled food, there’s no need to hold the mustard.

What Is Mustard Algae?

Mustard algae is a chlorine-resistant algae that often resembles dirt or sand on pool walls and floors. It creates compounds that act as a defense mechanism against the oxidation of sanitizers. This means that it can survive even when your sanitizer is top-notch. It’s actually a form of green algae, but think of it as its older, more athletic brother.

Where Does Mustard Algae Come From?

Mustard algae is often found in warm climates, and tends to live in fresh water—but for your pool, it’ll happily make an exception. It might make its way into your water on the backs of debris such as leaves and twigs, phosphates, or even pollen.

If your pool chemistry is off, especially the pH or alkalinity of your pool, mustard algae is more likely to move in. And if you keep the water very warm using a powerful heat pump, you’re more susceptible to all types of algaes, so it’s especially important to stay on top of your pH and alkalinity.

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Why Mustard Algae Is a Problem

Since mustard algae is chlorine resistant, it’s as tenacious as a cockroach. And like these horrific insects, it can survive outside of conditions you would think possible—namely, outside of the pool. Mustard algae can cling to bathing suits, pool equipment, anything that might come back into the pool or be exposed to water sometime soon.

If you have mustard algae, you can be certain that your chemical balance is out of whack. All forms of algae are known to exhaust your sanitizer. Once your sanitizer uses up all its juice on algae, it doesn’t have any left for the other microscopic bacteria in your pool, including the kind that can potentially make you sick. And if that microscopic bacteria is allowed to run amok, the rest of your chemicals are going to be harmed in the process.

Without the right chemical balance, your pool water is no longer safe for swimming. And safety is a big deal. That’s why it’s important to get out algae as soon as you see it, and pause swimming in your pool until you can be sure it’s gone.

Put It to the Mustard Algae Test

Mustard algae doesn’t look like its green cousin. It’s not particularly slimy or clingy, which is why it often gets mistaken by pool owners everywhere for sand, dust, or a pesky metal stain.

To be sure that what you’re dealing with is actually mustard algae, give those golden spots in your pool a little push with a brush—preferably a pool brush that’s antimicrobial. If it stays put, it’s probably a metal stain, which you’ll still want to get out of your pool. If it moves around but doesn’t cloud up, it’s probably sand entering your pool through your sand filter (if so, that’s a problem, too). If it immediately disperses into a cloud, you’ve got mustard algae on your hands.

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How to Get Mustard Algae Out of Your Pool—In Seven Steps

Ready to get that mustard algae out of here? Remember, it’s wreaked havoc on your chemical balance, so we’re going to focus heavily and restoring that balance—and at the end of this process, we’ll use a perfect chemical balance as evidence that you’ve sent mustard algae packing. Just follow these steps and you and your water are in the clear.

1.Don’t Just Prepare the Water

Remember, mustard algae is particularly persistent because it can live outside of the water. There’s no point in just treating the water and allowing the mustard algae to reenter the pool party on a swimsuit or pool floatie.

First, take care of the stuff outside of the pool that mustard algae might be clinging to. Machine wash your swimsuits, disinfect and clean pool toys, and put any pool brushes, hoses, or other equipment in the shallow end of your pool—that way when we shock your water, we’re also sanitizing these tools.

2. Brush, Brush, Brush

Next, it’s time to brush—and I mean really brush. Mustard algae spores might be clinging to your walls completely invisible to the naked eye, so you’re going to want to brush the entirety of your pool walls and floors. In order to save yourself a chunk of time—and back and hip pain—I highly suggest you use the antimicrobial patented 360-degree pool brush. It was developed by longtime pool servicers to be the easiest to use ever. When that’s done, manually vacuum your pool. A robotic cleaner might reintroduce mustard algae in, so it can be tabled (and disinfected!) for now.

3. Test Your Water… & Just Keep Brushing

Using water test strips, balance your pool water. You’ll want to give special attention to your pool’s pH and alkalinity levels. For pH, an optimal range is 7.4–7.6 with an ideal level of 7.5. Alkalinity should be between 100 and 150 parts per million (ppm), with an ideal level of 125 ppm.

Once you’ve rebalanced your chemicals and your levels are good to go (testing again is how you know this for sure), brush the entire pool again.

4. The Shocking Solution

Triple-shock your pool. That means three pounds of pool shock for every 10,000 gallons of water your pool contains (and if you’ve never calculated your pool size before, now is the time).

Once you’ve determined how much pool shock you need based on how many gallons of water your pool contains, walk slowly around the perimeter of the pool and distribute the pool shock into the water. You want to do this at dusk or else the sun will stop the shock from working, so do this next step and then call it a day.

5. Keep Going!

Remember, this type of algae is known to be stubborn and persistent. To really make sure you’re getting it out of your pool, you’ll need to dedicate a few days to the process. For about 72 hours following your shock treatment, keep brushing the pool and rebalancing your chemicals at least once a day.

6. Shocking… Part Two

After 72 hours or so, shock your pool again—but this time, make it the normal dose. That’s one pound of pool shock for every 10,000 gallons of water your pool contains. The same guidelines about applying shock after dusk apply, and you’ll want to wait until the shock has cleared in your pool water before moving on to this next and final step.

7. Make Sure Mustard Algae is Outta There!

The secret to making sure the mustard algae is gone? Here’s a hint: it doesn’t require a microscope, but it does require a water test strip. Check your chemical balance! If it’s stable and on the mark, you’ve handed your mustard algae an eviction notice. Great work.

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Bye, Bye, Algae

It took a few days, but you were even more persistent than your yellow algae and eradicated it from your pool. Good on you! Now you can lean into the good kind of mustard in your life: bright yellow swim trunks, or even the sauce on your next backyard hot dog. Enjoy.


Related articles:

Sand in Your Pool? Here’s Why—And How to Fix It

Chlorinate Your Pool in Three Easy Steps