How to Get Rid of Green, Yellow, and Black Pool Algae

Updated on August 7, 2019
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Barack is an expert pool chemistry guy and experienced online-based pool maintenance assistant via in-depth articles that top search results

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How to Get Rid of Green, Yellow, and Black Pool Algae

If a swimming pool is not well-chlorinated, it may develop three different colors of algae: Green (may appear dark blue), yellow(mustard), and black.

Green algae is the most common type and also the easiest to remove. All types of algae may cause a number of health hazards ranging from a slippery coating on skin and pool walls to a harmful bacteria growth in your pool.

Why Is My Pool Water Green?

  • If you are asking this question, it is because you have green algae in your pool.
  • Green (dark blue) algae forms due to poor pool sanitization (chlorination) and poor filtration.
  • This type is the easiest to get rid of compared to yellow(mustard) and black algae.
  • It floats freely on the surface, which makes the water look green.
  • It will sometimes stick to the pool walls, but the good news is that it is easier to brush off than the yellow or black types.
  • To get rid of it, you need to do a light shocking using liquid chlorine (see instructions below).
  • You can use algaecide preventatively once a week, especially during hot and humid seasons to keep algae away.

Why Is My Pool Water Yellow or Orange?

  • This means that mustard or yellow algae is growing in your pool.
  • This type looks like sand or pollen. It's very sticky and adheres to pool walls that don't get lots of sunlight.
  • It is chlorine-resistant, very stubborn, and in some cases, it will grow even on pool equipment, toys, floats, or bathing suits.
  • You cannot get rid of this type with a small amount of chlorine. You will need to deeply shock the pool using a high dosage of chlorine and clean all affected equipment with chlorine, as well. See instructions below.

What Is That Black Stuff In My Pool?

  • Black algae is the least common and the most difficult to get rid of.
  • It has a very strong defense mechanism and deep roots that make it very hard to remove. It will normally look like small, dark black spots on your pool walls.
  • To get rid of it, you will need to do heavy brushing, vacuuming, and chlorination. (See instructions below.)

What Causes Algae to Form in a Pool?

The main cause of algae in a swimming pool is low free chlorine(FC) level. Algae is rampant in hot and humid climates or during the summer, simply because chlorine reduces faster as water temperature increases leaving your pool vulnerable to algae and bacteria.

As such, the only way to keep algae and bacteria away is to maintain recommended FC level of 3ppm all the time by adding chlorine whenever it drops below 3ppm and this should be done on a daily basis. But don't think, even for a moment, that maintaining chlorine levels is enough: You need to maintain an appropriate balance of all chemicals in your pool because they affect how long chlorine lasts in your water.

Most chemicals used in the pool, such as pH chemicals, sanitizers, alkali, calcium, stabilizers, and more, will affect how chlorine works and how it is consumed. For this reason, the only perfect way to avoid algae or cloudy water is to constantly maintain the correct chemical balance in your water, which means using an accurate pool water testing kit.

What You'll Need to Remove Pool Algae

  1. pH adjusting chemicals
  2. Liquid Chlorine shock(recommended)
  3. Algaecide(for weekly maintenance)
  4. Vacuuming machine
  5. Backwash hoses
  6. A working filter system
  7. Large leaf net
  8. Accurate swimming pool water test kit
  9. Pool brush

6 Essential Steps for Removing Pool Algae

First, you need to begin troubleshooting the problem one step at a time until you end up with clear, blue water. For yellow and black algae, it is a long process and cannot be achieved in a day or two, but for green algae, you may solve the problem in a matter of hours.

Step One: Brush All the Surfaces Inside Your Pool

Yellow and black algae will require a heavy brushing of pool walls and floor, while green algae may only need a light brushing.

Step Two: Remove Large Debris

Large and visible debris should be dealt with first. Here, you will need to use a large leaf net instead of the hand skimmer. Note: A large leaf net will stir up the entire pool and move debris that has settled on the bottom. As a result, the water may look dirtier, but it will eventually settle down with time.

Step Three: Vacuum to Remove Small Particles

Be cautious not to use vacuuming equipment if the water still has large bits of debris floating in or settled on the bottom of the pool. This will block your pipes and filter. Note: Only vacuum your swimming pool when you can clearly see its bottom to ensure that no large debris still remain.

Step Four: Correct the pH Level

At this point, your water may still be an off-color, but it should be free of large and small particles. Now, you need to test and establish the pH level before adding any chlorine, especially when using Calcium Hypochlorite(Cal hypo) also known as super shock or altra shock that comes with high pH levels.

Depending on the chlorine you are using, the pH tends to rise when chlorine is added in a swimming pool; if you are using Calcium Hypochlorite(powder) chlorine, you need to keep your pH as low as possible, not higher than 7.2.

Another side effect of Calcium Hypochlorite is that it has lots of calcium and may cause calcium scaling and this may hugely affect your pool chemistry or damage your pool, and that is why liquid chlorine(Sodium Hypochlorite) is the best option to fight algae where a lot of chlorine will be used and it's also active and works faster than powder or tablet chlorine. Reducing the pH may be necessary as a general rule because chlorine will more effective in low pH that high pH levels.

Step Five: Shock the Swimming Pool and Maintain Shock Levels.

This step involves adding chlorine to raise your FC to shock level in relation to Cyanuric acid level in your water, and maintaining FC at that level by adding more chlorine when FC falls below maximum target range to be able to kill all agae, bacteria, and ammonia.

It's important to mention that killing algae is an experimental process; therefore, results may not appear after one day. Clear pool water is usually achieved after a few days of thorough shocking and may go upto a week depending on how strong the algae has hit your pool.

Chlorine levels in the water may be high as a result of this process; however, it will reduce with time. Don't worry too much. The main thing here is to focus on the clarity of the water.

Depending on the type of algae and how strong the algae has affected your pool, you may not know the exact amount of chlorine your pool will need to get rid of the algae, as such, be prepared with sufficient amount even if you will keep the rest for later use.

If you can avoid using Cal hypo chlorine shock and use liquid chlorine(sodium hypochlorite) the better because you will have quick results and the easier your work will be when balancing chemicals afterwards.

How to Add Liquid Chlorine to Kill and Get Rid of Algae

Depending the level of your Cyanuric acid(Cya) in your water, raise your FC to shock level. The recommended Cyanuric acid level to avoid using much of chlorine is always 30ppm, which requires 12ppm FC shock level and FC should not go below 6ppm which is the maximum range.

Very important; whichever shock level and Cyanuric acid level you choose to work with, test your FC after every hour and raise it back to the shock level when it reaches the maximum FC range, which is simply the level your FC should not go below in order to be able to kill all algae.

Cya of 40ppm will need 16ppm FC and shouldn't go below 7ppm, Cya of 50ppm will need 20ppm FC and should not go below 8ppm, and Cya of 60ppm will need 24ppm FC and shouldn't go below 9ppm until your water is free of algae.

Spare enough time to kill and get rid of algae because the process of testing FC and raising it back to the shock level might take a couple of days, and sometimes it may go up to a week or so depending on the algae you are fighting and how strong it has established itself in your pool.

Very high FC levels is very destructive to pool parts and if your Cyanuric acid is higher than 30ppm, you will have to lower it first by draining and refilling 1/4 to 1/2 of your pool water.

For sodium hypochlorite with 12.5% chlorine like Chloro Guard Liquid Chlorine, 10 fluid ounces will raise FC by 1ppm in a 10,000 gallons pool, as such, you will need around 120 liquid chlorine ounces to raise FC by 12ppm in a 10k pool if it's zero. Chloro Guard liquid chlorine is one of the best liquid chlorine in the market due to it's fast action and doesn't alter pH and calcium levels in your water.

After raising your FC to 12ppm with Cya of 30(recommended), take FC reading in an hour's time and raise it back to 12ppm if it falls below 6ppm, which is the level it should not go below in order to kill all algae, repeat the process until you can't see traces of algae and this might go on up to a week or so.

If it takes more than a week of adding chlorine to shock level but you can still see some spots of algae, find some Trichlor and apply it on spots that are still affected by algae but don't use a lot of as small amount will be effective, too much of Trichlor will raise your Cyanuric acid levels and it might get off the charts. I recommend you use Trichlor by In The Swim which is very strong and has 90% chlorine and the remaining algae will disappear.

Finally, when your FC get down to 3ppm, balance all your pool chemicals starting with the pH(7.2-7.6), Cyanuric acid(40-60ppm), TA(80-120ppm), and CH(200-400ppm).

Using Calcium Hypochlorite

If you are aware of the side effects of using Cal hypo including raising pH and Calcium levels, and you are ready to tackle possible pH and Calcium scaling that might arise in the process, add 3 pounds of cal hypo for every 10k gallons pool to be able to kill algae, and follow the same procedure of testing and raising FC till it settles at target shock level, then balance all the chemicals when FC finally hit 3ppm.

As a general rule, 1 pound of cal hypo is enough to raise your FC by around 3ppm for 10k gallons pool and FC may last up to a week before completely depleting since it has high chlorine percentage of above 60% and doesn't melt faster in sunlight.

Step Six: Run the Filter 24 Hours a Day

The more you run the filter, the faster the water will clear up. Also, remember that dirty swimming pool water will clog the filter system more frequently during this process, particularly the DE and cartridge pool filters. Because of this, you need to backwash your filter every day to ensure that it operates effectively during the process.

Note: The more the filter runs and the more it is backwashed, the faster the water will clear up.

If you have the DE filter, this process may be shorter since the DE filter is more efficient than the sand or cartridge types, but you need to remember to add a new DE after every backwash to facilitate the process.

Whichever filter you are using, ensure that your system is functioning properly. If the filter system is not working properly, the pool water will never clear up, even after all this effort, and you will have to replace or repair the filter.

A cleared swimming pool will be blue in color; while a clearing pool water will be light green or cloudy white.
A cleared swimming pool will be blue in color; while a clearing pool water will be light green or cloudy white. | Source

Important Tips

Chlorine levels:

  • Free chlorine (FC) is responsible for attacking bacteria and other contaminants. As the FC is used up, combined chlorine (or chloramine) is formed, which cannot kill bacteria and other harmful organisms. The higher the combined chlorine in the water, the higher your chances of having algae.
  • There are a number of chlorine shocks you can buy and use in your pool; whichever you choose, ensure that FC is always maintained at 3 ppm. But remember that calcium hypochlorite alters your pH and Calcium Hardness, and tablets chlorine raises your Cyanuric acid in the long run, and the best option is always sodium hypochlorite or liquid chlorine.
  • It is recommended to maintain combined chlorine levels always below 0.5 ppm or at 0.0 ppm, if possible. To reduce chloramine levels, you must shock the pool on daily basis using liquid chlorine.
  • If you have an outdoor swimming pool, you can use cyanuric acid as chlorine stabilizer to help prevent UV rays or hot water temperatures from consuming free chlorine at a higher rate.

pH Levels:

  • Ensure that you always maintain the correct pH level. It should always be between 7.2 and 7.6, with 7.4 being the ideal reading.
  • To control the pH, use either pH Plus (pH increaser) to raise it and muriatic or sulfuric acid or pH Minus (pH reducer) to lower it.

Alkalinity:

  • When total alkalinity (TA) increase out of range (between 80 ppm and 120 ppm), pH and calcium will increase to destructive levels.
  • Ensure that you lower TA to recommended levels when it goes high to avoid issues with pH and calcium scaling.
  • It is always essential to frequently test your water and take chemical readings to ensure that it is chemically balanced. If possible, testing should be done on a daily or weekly basis depending on which chemical you are testing.
  • Rather than guessing the chemical level by matching colors with several brands of test strips, I use a digital pool water test kit. If you can find an accurate and easy-to-use kit like the LaMotte ColorQ Pro 11 digital water test kit, it will save you a lot of time as it is easy to use and very effective. It contains a number of tests including pH, free, combined, and total chlorine levels, bromine, calcium hardness, total alkalinity, metals, and cyanuric acid.
  • To avoid troublesome mathematics, after getting the correct values of all chemicals, I use a pool calculator to find appropriate amounts of chemicals to add. How easy can pool chemistry get?

Filter:

  • Your filtration and circulation system plays a very important role in pool health. A faulty filter and poor circulation system will cause dirty and stagnant water, which may bring about algae, ammonia, cloudy water.

Pump:

  • You need to ensure that all pumps are working properly. Regularly clean, change, and backwash your filter when needed.

Which Pool Water Test Kit do You Use?

See results

Conclusion

This comprehensive DIY guide should help in your efforts in eliminating algae. It is also important to note that maintaining a pool is cheaper than paying to rectify a problem.

Swimming Pool maintenance

How regularly do you check and maintain your swimming pool status?

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

  • My pool water is crystal clear but I have stubborn patches of mustard algae. How do I get rid of the patches?

    Mastered algae is very tough and you need to scrub or brush off most of it and remove any loose material you can see in your water using hand skimmer or large leaf net.

    After brushing and vacuuming your pool, depending on the level of your Cyanuric acid(30ppm most ideal), add lots of liquid chlorine(sodium hypo) in your water to raise FC to maximum shock level that can kill algae, 12ppm is the shock level when Cyanuric acid is 30ppm.

    Take your FC reading in 1 hour and if it drops below 4ppm, raise it back to 12ppm and take the reading again after an hour, repeat the process until your FC is stable between 4 to 6ppm and mastered algae will disappear without coming back.

    Finally, leave your pool running so that your FC comes to 3ppm, then balance your water starting with the Cyanuric acid at 40ppm, pH, TA, and CH.

  • My pool water is well balanced and very clear, but there is a greenish coating adhering to the floor and walls which cannot brush off. Could it be pollen?

    The greenish coating is algae. You need to get rid of algae using Algaecide, which you can find on Amazon through a link in this article. If Algaecide does not work within 24 hours then you need to super shock the pool using plenty of liquid chlorine till the greenish coating disappear.

  • What causes ammonia in pool water?

    Ammonia finds its way in swimming pool water as a result of bacteria coming in when FC level goes below recommended levels, leaving high Cyanuric acid that is converted to ammonia by bacteria. Also, you can get ammonia as a result of decayed leaves and other materials from plants being swept or dropped into the pool. You can keep ammonia away by cleaning leaves and other plant materials from your pool as soon as they get into the pool. A pool that is not properly chlorinated has a higher chance of having ammonia so it is always good to watch over chlorine levels to keep ammonia away from your pool.

  • I had black sooty like residue on the bottom of my vinyl pool. So I super shocked it with liquid chlorine. And now the residue turned green. It’s not stuck on the bottom it’s like when you move it floats away like a cloud. I’ve tried vacuuming it. And it just comes back. What do you think it is and what should I do? The water is crystal clear and ph is 7.6

    This could be either green or black algae. If your free chlorine(FC) level is below 2.5ppm or combined chlorine is above 0.5ppm, this could be algae. To be sure you have algae and not metal stain, do overnight chlorine loss test(OCLT) by adding chlorine to raise FC to 3ppm in the evening and take reading the following morning, if you lose 1ppm and above in the morning, that is algae and you need to lots of liquid chlorine to kill it all. Raise your FC to a shock level in relation to Cyanuric acid level in your pool i.e if your Cyanuric acid is 40ppm(recommended), the FC shock level you should reach is 12ppm and maintain it thereby raising your FC at least twice a day until you see no more signs of algae.

  • Can the wrong size pool pump cause algae?

    Not really, algae is caused by low free chlorine (FC) level as a result of poor pool sanitization or chlorination. Your FC should always be 3ppm to keep algae and bacteria away from your pool. However, the wrong pool pump size may cause poor filtration, which can lead to dirty or cloudy water and that may keep bacteria causing algae or ammonia in your pool.

© 2014 Barack James

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