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The golden yellow mustard algae are stubborn molds that will ruin your pool faster than you can spell xanthophytes (the name of the mustard algae microbe family).

These yellow algae in the pool rarely appear, but they are very unruly. Not to worry, though. With a few simple steps, you can eliminate these foreign bodies from the pool and also prevent them from returning.

But first, we need to learn about yellow algae in the pool.

What are mustard algae?

In a way, yellow mustard algae are siblings of the normal green algae that clings to pool walls like a nasty, slimy mass. Mustard algae are similar to these, but – well – yellow. In fact, there are other differences that require different treatment.

Mustard algae is often mistaken for dirt, stains or sand. However, the yellow algae is a member of the Xanthophyte family of microbes. Unlike their green counterpart, mustard algae are chlorine resistant and much more persistent.

Yellow mustard algae like to attach themselves to pool walls and turn the pool water yellow. What is particularly dangerous about this type of algae is that they can survive outside the swimming pool. Their alien visit also makes its way onto pool equipment, toys, your swimwear, ladders and stairs.

Many people also wonder whether algae in the pool is dangerous or not. However, the question can be answered briefly: algae that colonize swimming pools are harmless to health, but due to the (sometimes strong) turbidity, the pool can become a dangerous trap for animals and small children.

Remove yellow algae from the pool in 11 steps

Yellow algae can be stubborn. However, there is no reason to panic. Get rid of the pest with these simple steps.

Step 1: Clean your swimwear in the washing machine

After you’ve noticed mustard algae in your pool, the first thing you should do is clean all of your swimwear in the washing machine. This will kill the algae that has settled into the fabric of the clothing and give you peace of mind that you won’t reintroduce the algae back into the pool.

Step 2: Clean the pool accessories

Now clean pool toys, swim and swap equipment, and the rest of the pool equipment with a chlorine-based cleaner. The chlorine (in the form of chlorine tablets, for example) ensures that algae is killed. A cleaner that is too aggressive, on the other hand, can attack plastics.

Pool accessories that are unwieldy or very bulky can also simply be placed in the pool. Instead of cleaning it by hand, the next steps disinfect it together with the pool water.

Step 3: Brushing and vacuuming

Once the preparations are complete, the actual cleaning can begin. First, you should use a pool brush to remove the yellow algae. Do this thoroughly to make vacuuming easier on yourself.

When vacuuming, you want to make sure the filter is on waste (“waste”). The algae could otherwise spread through the filtration system.

When you vacuum, you lose water, but also get rid of the algae once and for all.

When the algae infestation is apparently eliminated, refill the pool water to the original level. You can use a manual pool vacuum for vacuuming, for example.

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Step 4: Test the pool water

A balanced pool chemistry is the starting point for the next steps. So pick up a test strip and check the levels.

With balanced water, the upcoming shock chlorination of the pool water will be more effective. For this, the values should be in these ranges:

  • pH: 7.0 to 7.4
  • Total alkalinity: 80 to 150 ppm
  • Chlorine content: about 3 ppm

For example, a too high pH would reduce the effectiveness of the pool shocker.

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Step 5: Correct the Pool Chemistry

If the measurements differ from the ideal values, you need to correct them. If the values are all OK, you can skip this step.

Use the two chemicals, pH-Plus and pH-Minus, to control the pH. The pH-Plus (pH-Minus) is a chemical that raises (lowers) the pH in the pool water.

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If the alkalinity (TA) is too low, you can add an agent to raise the level. If the alkalinity level is too high, it can only be corrected by (partially) replacing the water or by pouring pH Minus all at once into the deepest part of the pool.

Note: Adjust the alkalinity first if you want to adjust the alkalinity and pH.

For a higher chlorine content, you simply need to add more chlorine granules. Over time, the concentration of chlorine will decrease again. So you don’t have to actively lower the chlorine concentration.

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Step 6: Brush the pool again.

Hey, we didn’t say this was going to be a breeze.

Once again, it’s time to reach for your trusted brush and remove the remaining algae from the pool walls and crush it. By doing so, you’ll make the pool shocker’s job a whole lot easier.

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Step 7: Perform a triple shock chlorination.

The yellow algae are slightly less susceptible to the disinfectant chlorine than their brothers – the green algae. For this reason, increase the dosage to kill this stubborn type of algae as well.

Choose three times the dosage for this purpose. Shock chlorination is performed after sunset, because the sun would burn the unstabilized chlorine of the pool shocker, reducing its effectiveness.

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Step 8: Run the filter and pump all day.

Next, you should reprogram your filter pump to run 24 hours a day. Remain with this programming until the algae is gone.

Step 9: Brush and test again.

Mustard algae is tough. Sometimes it seems like you’ve completely removed the algae, only to have it reappear.

Again, meticulously walk the pool walls and brush any last bits of algae from the walls. Focus on the areas where the algae first appeared.

It’s also time to test the pool water. Check to see if the pH, alkalinity and chlorine levels are in the green. In particular, the chlorine (or disinfectant you are using) is important to prevent the algae from re-germinating.

Step 10: Shock chlorination the second

Time for déjà vu of a different kind. Don’t give the mustard algae that survived all the ordeals a false sense of security, after all, but perform shock chlorination again.

This last time, a simple dose of pool shocker will suffice. Add the chemical to the water again after dark to wake up the next day with an algae-free pool.

Step 11: Congratulations!

Congratulations! The pool is now free of algae.

Check the water chemistry again and adjust if necessary.

Note: In rare cases, the water may be cloudy. However, this is only a side effect of the pool shocker. You can solve the problem immediately with water clarifier, or just wait a bit.

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Tips to keep mustard algae away.

If you’ve had to do the work once, you’re definitely looking around now for the proper precautions to make sure it doesn’t happen again.

We have compiled a short list of tips. If you pay attention to all of these tips, the risk of another algae infestation is minimal.

  • Shock chlorinate the pool water once a week – after dark.
  • Make sure your pool chemistry is balanced. Alkalinity, pH and chlorine levels are the most important metrics.
  • Clean the pool and pool accessories regularly and thoroughly.
  • Wash bathing suits and trunks that you have been swimming in lakes, the ocean or rivers with before wearing them in the pool.
  • Disinfect pool toys that have had contact with lake or ocean water with a chlorine-based cleaner before they end up back in the pool.

Mustard doesn’t belong in the pool, but on hot dogs. Keep the mustard in the kitchen with a clean pool and a pool maintenance plan, so you can relax and swim without worrying about such pesky invaders.

Larry has been a true water rat since childhood. Pure pleasure turned into a passion. That's why he is the first point of contact for friends and acquaintances when it comes to pool-related problems. He is an integral part of the PoolHandbook editorial team.