How to Get Rid of Mustard Algae in Your Pool

If you’ve ever seen your pool when it’s yellow and cloudy, you know it’s not a good look. So, what happened? The likely culprit is mustard algae.

Although mustard algae can be a challenging algae bloom to rid your swimming pool of, it’s entirely doable. All you need are the right tools and a solid foundation of information to start the cleaning process.

Whether you’re interested in preventing pool algae as a pool owner or are currently struggling with a mustard algae outbreak in your own swimming pool, rest assured our handy guide will help you clear up this annoying problem. You’ll learn what mustard algae is, how to kill it, and how to prevent mustard algae in the first place.

So, What’s Mustard Algae?

Mustard algae (also called yellow algae) is like the cousin of green algae, but more commonly seen in southern climates like in 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.

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

What Causes Mustard Algae in the First Place?

As annoying as it is, there are a few causes behind that icky mustard algae in your pool. The algae spores first enter your swimming pool, though either the elements (like wind or rain) or through swimwear or equipment that are contaminated with the spores.

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

  • Lots of warm, sunny days
  • Having high nitrates or carbon dioxide levels in your pool
  • Pool water that’s out of balance
  • Not sanitizing or filtering your pool properly
  • Forgetting about circulation

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

Mustard algae is relatively easy to identify. It’s more likely to return than other types of algae and is chlorine resistant. If you were to brush it away, it would likely grow back in the exact same place. However, this doesn’t mean mustard algae makes your swimming pool water hazy or murky — a pool infected by mustard algae will usually be clear. If you’re pool is cloudy, read my guide on how to clear cloudy pool water.

Mustard algae isn’t the “slimy” algae you might be used to, and because of this, it can be easily mistaken for stains, sand, or dirt. It’s also usually found at the sides or bottom of the swimming pool.

How to Eliminate Mustard Algae in Your Pool

Removing mustard algae can seem like a daunting task, but there are some simple steps you can follow to get everything cleaned up and looking crystal clear again.

1. Remove Everything From Your Pool and Clean It

A good first step is removing everything from your swimming pool. That means any items, floats, and toys. Disinfect these items with a solution that contains chlorine (but not bleach). You’ll also want to wash all your bathing suits and other swimwear thoroughly.

2. Brush and Vacuum the Mustard Algae

Brush the walls of your pool to remove any algae hanging onto the walls and let everything settle before manually vacuuming. Ensure your filter is set to waste mode so the vacuumed water doesn’t release back into the pool. Finish off by topping your pool with fresh water to replace any water that was vacuumed.

3. Balance Your Pool Water

A crucial step after brushing the algae from the walls and floor of your pool is to balance your pool water. To do this, you’ll need to do two things. First, clean and check out your swimming pool’s filter system. Then make sure the pH levels are correct.

You can also go ahead and refill your pool with new water. Using a manual pool vacuum, suck up any and all debris from both the swimming pool floor and the walls. Ensure you’re using the “waste” option as opposed to the “backwash” setting — this will help you get rid of much of the algae. Afterward, you can use a brush on your pool walls to get any of the leftover algae.

Once you’ve done that, you can make sure your pH levels are correct. Using a pool test kit, ensure your alkalinity level is 120-150 parts per million and that your pH level is 7.4-7.6. Algae loves water with higher pH levels, so it’s essential to keep the balance.

4. Brush Again

Before you shock your pool water, make sure to brush your pool walls and floor again. If you have a vinyl pool, use nylon brushes; if you have a plaster pool, use steel bristle brushes. To continue the process, you’ll want to add some algaecide — which you can find online or at a hardware store — and brush your pool one more time.

5. Shock Your Pool Water

Time to shock your pool. Follow the instructions on the shock that you pick up and multiply everything by 3 to make sure you eliminate all the mustard algae. A good general rule of thumb is about 3 pounds of shock for every 10,000 gallons of pool water. You can use my pool volume calculator to figure out how many approximate gallons of water are in your pool if you don’t know already. And remember to shock your pool at dusk! Sunlight can make your shock way less effective.

6. Circulate That Water!

You can encourage circulation in your swimming pool by using the automatic cleaning settings. You should also run the filter for at least 24 hours.

7. Keep Brushing and Adjusting Chlorine Levels

Over the next 3 days or so, keep brushing your pool walls to get all that mustard algae floating freely in the pool water for the chlorine shock to do its job. Also keep testing and adjusting your chlorine levels to make sure it stays highly effective at killing everything off.

8. Ensure the Mustard Algae is Gone

The water should be clear but test the balance to be sure. Only put the objects like floats and toys back into the swimming pool when the algae go away. Mustard algae can be very hard to get rid of once you have it, so the minute you notice it, start treating it. If the first shock doesn’t work, keeping shocking. Also, note that it will latch onto anything that is in your pool. Sanitize items well so they don’t reintroduce algae into the pool.

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How to Stop Mustard Algae from Taking Over Your Pool

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

There are a few things you can do to stop the mustard algae from taking over your swimming pool, however. One of the easiest ways — especially when it’s warm outside — is to use your pool regularly.

Ensure your alkalinity, pH, chlorine levels, and sanitizer levels are always in the appropriate ranges.

It’s also critical that you, as a swimming pool owner, keep your pool clean. That includes cleaning equipment and items like pool ladders and toys.

Make sure you practice proper sanitation techniques and run the filter for about 8-12 hours daily, during all seasons. Every week, you need to brush and vacuum your pool’s surfaces, like the walls and bottom. You can invest in a robotic pool cleaner to help you out.

Adding algaecide and shocking your swimming pool on a weekly basis can also help prevent mustard algae. Remember, it’s best to shock at night.

It’ll also go a long way to ensure you get into the habit of cleaning everything. 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 these things on a regular basis.

If your mustard algae keeps coming back again and again, it’s time to drain the pool. Use a technique like acid or pressure washing. This will kill any algae roots that could be embedded in your pool. After this, clean or change out the filter.

Although mustard algae can be incredibly irritating — not to mention one of the most difficult to rid your swimming pool of, thanks to its chlorine-resistant properties — don’t lose hope if you spot that yellow, cloudy bloom in your swimming pool.

So long as you follow the proper steps to rid your swimming pool of its algae and implement the preventative tips above, mustard algae will soon become a problem of the past.

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