Mustard algae, a stubborn and unsightly growth, can quickly ruin the aesthetic appeal of your swimming pool, leaving behind a thick, yellowish-green layer. It’s not only uninviting but also poses health risks such as skin irritation, respiratory problems, and even infections like UTIs to swimmers. In this guide, we’ll discuss what causes mustard algae to form in pools and how you can remove it.

Causes of Mustard Algae Build-up in the Pool

 

1. A faulty pool filter

A malfunctioning filter allows debris, such as leaves and dirt, to build up in the water. This debris is often rich in nutrients like phosphates and nitrates, which serve as food for mustard algae. When the filter isn’t functioning properly, it fails to remove this organic matter, creating an ideal environment for algae to thrive.

 

2. An ineffective pool pump

The pool pump is crucial in maintaining adequate circulation, pulling water through the filter, and redistributing it. So, if a pump is faulty or underpowered, it creates stagnant areas in the pool where algae can settle and grow.

 

3.Too low or too high chlorine levels

Since Mustard algae is chlorine-resistant, it can be tricky to understand the impact of low or high chlorine levels on it. While it’s true that chlorine can’t effectively kill mustard algae, it prevents it from thriving. So if the levels fall too low, not only will it continue to live but rapidly proliferate.

Conversely, adding excessive chlorine to your pool to get rid of Mustard algae can cause a “lock” effect, rendering it altogether ineffective due to the chemical imbalances it causes. For effective control, maintain chlorine levels between 1.0 to 3.0 ppm using a pool water test kit.

 

4. Disrupted pool water chemistry

Chlorine levels aren’t the only concern – pH, alkalinity, and calcium hardness also affect your pool’s health. 

An imbalance in your pool’s alkalinity causes spikes in the water’s pH. Consequently, if pH levels drop low, it can lead to erosion on pool surfaces, thus providing algae an ideal space to flourish. Whereas high pH makes chlorine ineffective, causing rapid algae growth. 

Calcium hardness also plays a role in preventing the water from becoming either too corrosive or too scale-forming. For best results, aim for the following ranges:

• pH: 7.2 to 7.6
• Alkalinity: 80 to 120 ppm
• Calcium Hardness: 200 to 400 ppm

 

5. Extreme temperatures

Since mustard algae thrive at high temperatures, summers are its favourite season to colonize pools. It’s also the same weather when pools are most used hence, high chances of its accelerated growth. Covering the pool to protect it from direct sunlight exposure can help keep the temperature down.

 

Steps to Remove Mustard Algae

Here’s a quick list of supplies needed to remove mustard algae from your pool:

• A pool brush
• Water test strips (for pH, alkalinity and FAC levels)
• Sodium carbonate (soda ash) to raise pH
• Sodium bisulphate (to lower pH)
• Pool shocking solution – calcium hypochlorite (cal-hypo granular) or sodium hypochlorite (liquid bleach)
• Water clarifier
• Pool vacuum (optional)

Once you have all the supplies at your disposal, it’s time to get started with the algae-removing process: 

 

1. Do the mustard algae test

To ensure that you don’t confuse mustard algae with a metal stain or sand, do a quick test. Take an anti-microbial brush and scrub the spot you think is home to mustard algae. If it doesn’t move, it is a metal stain; if it disperses but doesn’t make the water cloudy, it’s sand. However, if it moves and turns the water in that area milky, it is mustard algae.

 

2. Wash your pool toys and swimsuits

The next step is to wash everything that has been in the pool or even close to it because this algae doesn’t need water to survive and can cling to your pool toys and swimsuits. Failure to clean these can bring mustard algae back to your pool easily at a pool party.

Also, put all your cleaning equipment at a shallow end of the pool so that when you later add the shocking chemicals, it sanitizes your tools too.

 

3. Thoroughly scrub and brush the pool

Now start brushing your pool using a pool brush – from the walls to floors, steps and corners. Do not leave any spot unattended just because a golden mustard algae stain isn’t visible to your naked eye – you never know where the spores might be hiding.

With this rigorous brushing, the algae will lose its grip and be pushed out into the water, where the shock will kill it.

 

4. Test the water pH

Before adding the shock, it’s crucial to check the water’s pH and alkalinity using test strips. The shocking solution is not really effective when the water chemistry is off, so getting the pH and alkalinity within optimal ranges is essential:

• • pH: 7.2 to 7.6
  • Alkalinity: 80 to 120 ppm

If your levels are outside these ranges, adjust them:

• • To raise pH: Add sodium carbonate (soda ash). About 6 ounces per 10,000 gallons raises the pH by roughly 0.2.
  • To lower pH: Add sodium bisulfate, which is safer than muriatic acid. About 5-7 ounces per 10,000 gallons lowers pH by 0.1.

Remember to always dilute acids in water before adding them to the pool to avoid damage.

 

5. Add the Shocking solution

Next, you’ll need to shock the pool, a process called superchlorination. This means adding much more chlorine than usual to burn through the resistant mustard algae. Use either calcium hypochlorite (cal-hypo granular) or sodium hypochlorite (liquid bleach) for this step.

To get the best results:

• • Shock the pool at night or dusk, when the sun won’t burn off the chlorine too quickly.
  • Run your pump and filter continuously for 24 hours after shock.
  • Stick to the manufacturer’s instructions for how much shock to use.

Since mustard algae is stubborn, this process might take a few days. Keep brushing the pool daily for the next 72 hours, testing and adjusting your chemicals as needed.

 

6. Clean the filter and vacuum the pool

After the shock has circulated for enough time (as recommended by the manufacturer), clean your pool filter. Backwash the sand for a sand filter, and clean the cartridge for a cartridge filter.

 If you haven’t changed the sand or cartridge in the last 2 – 3 years, we recommend doing that because there is a high chance that your filtration system has gone ineffective.

Once your filter is good to go, turn your pump back on and check the Free Available Chlorine (FAC) level using a test strip. If it’s below 4.0 ppm, add a water clarifier. It will bind the dead algae into larger particles that are easier to filter out. If any algae debris remains, vacuum the pool using the filter’s “waste” setting to fully remove the remnants.

 

7. Test the chemical balance again

The final step is to test your water’s chemical levels one last time. Check that the pH and alkalinity are back within safe ranges (pH: 7.2 to 7.6, alkalinity: 80 to 120 ppm). Once the water chemistry is balanced, you should know that the mustard algae is gone for good and your pool is ready for use again.

 

How to prevent mustard algae from coming back into your pool?

By adhering to these dos and don’ts, you can significantly reduce the likelihood of mustard algae returning to your pool:

Do:

• • Run the filter for at least 12 hours daily to circulate water and remove impurities.
  • Clean your pool filter regularly to ensure no debris buildup.
  • Brush walls and vacuum regularly to remove spores and debris.
  • Keep chlorine between 1-3 ppm and test water frequently.
  • Shock the pool every 2 weeks or after heavy use to kill algae spores.
  • Maintain pH between 7.2-7.6 and alkalinity at 80-120 ppm for chlorine efficiency.
  • Add algaecides specifically made to fight mustard algae or cleaners containing potassium Tetraborate, chitin and phosphate removers as part of your routine maintenance.

Don’t:

• • Let water sit stagnant for extended periods, as it creates perfect conditions for algae to thrive.
  • Bring swimsuits and toys to the pool unless they have been thoroughly washed.
  • Leave your pool uncovered for an extended time under sunlight on high-temperature days.

 

Takeaways

Mustard algae can be tough to get rid of, but with the right steps and regular maintenance, you can remove it and keep it from coming back. By keeping your pool’s water balanced, cleaning often, and using the right chemicals, your pool will stay clear and safe. Plus, regular brushing, good filtration, and the use of algaecides and specialty chemicals in your maintenance routine will help keep your water clean and algae-free.