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Black Pool Algae Problems

A problematic type of resistant pool algae.
The Pool and Spa Informational Website or

Treating Resistant Pool Algae Problems.


Scroll down to browse through some archived SWIMMING POOL questions and answers.  Please click the Pool Problems Link, on top of every page, to access a complete listing of Pool Problem subjects, an alphabetized Website Table of Contents, Pool Equipment Information, About Alan Biographic Material and a Pool Glossary. Use the other links to access additional subject information. More information about some new and unique products, for pools and spas, can be found by visiting The Website Store. You'll never know what you'll find and that's always fun. Be better prepared and avoid costly problems!


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Optimizing chemistry, improving circulation and eliminating phosphates!!!
A ColorQ, All-Digital Water Tester can perform all of the common pool water tests, eliminates the color-matching and guesswork.  There is a model, for every pool testing need.  Reliable water testing will help solve and avoid problems.  Better Circulation helps make everything work more effectively. The Circulator is a return jet replacement fitting, that improves filtration and eliminates the dead zones, that promote algae growth. Phosphates and Nitrates can increase the growth of algae and make treatment more difficult, as both are vital plant nutrients.  Nitrate removal is not practical, but phosphate removal is easy enough to do.  Adding Pool Refresh Total Trap will allow you to vacuum and filter out phosphates and should make algae control more effective.
ColorQ All-Digital Water Testers The Pool Circulator Boosts Circulation How to eliminate phosphates
ColorQ 2X Testers are Bluetooth and can be used with the FREE WaterLink Solutions HOME App. The Ciruclater replacement return jet fitting improves pool water circulation. Pool Refresh TotalTrap removes phosphates and metals.
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If you have a pool or spa water testing need, we should have the product.
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Alternative Pool Water Sanitizers
A Salt Chlorine Generator is definitely a better way to do chlorine.  Salt chlorine generators are highly automated and give you better control.  The salt level is about that found in human tears.  In-Line and no-installation-required models are available.  Adding an Ultraviolet Sterilizer kills virtually all microorganisms passing through the cell.  While it must be used with a persistent sanitizer, such as chlorine, it reduces the amount of chlorine required, to maintain an optimum level.  Magnetic Water Conditioners help solve scaling problems, due to high levels of calcium hardness and are available in 4 models.
Salt Chlorine Generators Above-Ground UV Sterilizer Magnetic Water Conditioners
NUVO Ultraviolet Sterilizers for Residential Pools. Magnetic Water Conditioner, for calcium hardness and scale issue.s
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If you have a pool or spa water testing need, we should have the product.
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How to treat Black Algae and Resistant types of algae?

Black algae is one of the most difficult varieties to control and eliminate. This algae can appear as a blackish discoloration or a tar-like deposit. In either case, it is a problem to remove because of the physical nature of this algae. It has, as its outermost surface, a layer of polymucosaccharide. This can act as a water repellent barrier and shield the underlying algae, from contact with the chemically-treated water. In addition to shock treatment and application of a polymer algaecide, it is recommended that the pH be dropped to 7.2, the circulation be directed towards the affected areas and a "quat" algaecide be added. The addition of the "quat" algaecide will not kill black algae, but it can act as a wetting agent, that will help the chemicals penetrate through the polymucosaccharide barrier. This regimen should be used with all types of algae that appear to be resistant to normal treatment, regardless of the color. Not all black stains are caused by black algae: heavy metal discoloration and plaster finish problems are other possibilities.  In addition to proper sanitation, good circulation is a must to help prevent algae growth in areas with stagnant water or dead zones. The use of The Circulator, as a replacement for standard return jet fittings, can dramatically improve circulation, better distributing sanitizer to all areas of the pool.  In addition to proper sanitation, good circulation is a must to help prevent algae growth in areas with stagnant water or dead zones.  Adding a 60% polymer algaecide will provide some backup algae control, especially important when chlorine or bromine levels bottom out. It can buy you some time, until the chlorine or bromine levels, can be replenished and restored to optimum conditions.   If problems arise, refer to the Pool Problems Page, as a source of problem-solving information, broken down into various categories.  Scroll down the page and click on the linked keywords, catch phrases or images, in the archived answers below, to access additional information, on that topic or product.

Do you know what's in your water?  If you're having problems, with sanitation or water clarity, testing allows you to better understand the chemistry and determine the cause of the problem.  Once understood, you can select the best treatment option.  Understanding the nature of the problem, should be step one.  For information about our full selection of testing options, visit our Test Equipment Store.

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Blackish Spots?

My inground pool has a marcite finish that is a few years old and is in very good condition. However, there are a few blackish colored spots on the bottom, in the corners and near the ladder. I have tried using a Siphoning Device on the end of a vacuum pole with some acid solution, but nothing has happened. I can't rub the spots off. Can this still be algae? Thanks.

Edward D, Bellingham, WA, 6/12/2020

The fact that the acid solution had no effect increases the likelihood that it is algae. Black algae, in particular, can be difficult. Fortunately you only have some spots and that can make treatment easier. Your Marcite finish allows for more direct
WaterLink SpinTouch Tester, for pools and spas. treatment. You have several options. Brush the surface to help expose the algae. Place a 3" trichlor tablet on top of a spot and allow to remain in place for at least a few hours. The tablets can affect some masonry finishes, so either test it on an inconspicuous spot or verify treatment suitability with the finish contractor. If improvement is seen, repeat elsewhere, as necessary. Another option is the use of a Granular Trichlor to sprinkle onto the spots. Shut off the filter first, so that the granules sink straight down. Use a brush to do the positioning. Another method is to attach a 3" trichlor tablet to a gadget (available in pool stores) on the end of a vacuum pole. This can be used to rub the trichlor onto the spots. NONE OF THESE OPTIONS ARE SUITABLE FOR OTHER THAN MASONRY FINISH POOLS - DO NOT PERFORM ON VINYL LINED OR FIBERGLASS POOLS.  In order to help keep the algae from returning, you should consider the use of a Polymer Algaecide. This product is effective against many resistant types of algae, doesn't foam and is non-metallic. To better assure proper overall pool water chemistry, visit a pool store that has a very reliable, professional lab such as a WaterLink SpinTouch Lab, rather than a less accurate test kit or strip reader.  If this website was helpful, in answering your question, please consider joining our E-Letter Mailing List.  I hope that this recommendation works out for you.

Sincerely. Alan Schuster, 6/13/2020

Black Swimming Pool Algae?

I have an 18,000 gallon vinyl inground pool and have a growing problem with a black deposit in the deep end of the pool. You can feel it on the surface - it is almost like tar. It started off as a spot and is growing. Is it black algae? What should I do? Your help will be appreciated. Thanks.

Brad R. Cherry Hill, NJ, 7/23/2020

I suspect that you have black algae. Black algae is a resistant type and it will take a regimen of steps to remove the problem. I hope that your pool has a main drain. If not, drop a vacuum head and hose into the deep end and attach to a skimmer intake. Good circulation, in the effected areas is important. Use a brush on the deposits to help loosen and
The Circulator improves pool water circulation. expose the algae. Add a quick acting shock treatment at the rate of 2 pounds per 5000 gallons, until a Free Chlorine Test reading of 5-10 PPM is achieved. Add an initial dose of a "Quat" Algaecide: this will not kill the algae, but will act as a wetting agent to help the chemicals penetrate the outer surface of the algae. Add an initial dose of a Polymer Algaecide: this product is effective in controlling resistant types of algae. Make sure that you are using a Free Chlorine tester. Test the pH of the water and lower to 7.0, in order to increase the effectiveness of the chlorine. Keep brushing the deposits, retesting the Free Chlorine and keeping the level at 5-10 PPM. Add more shock at the rate of 1 pound per 5,000 gallons, as needed, to keep the Free Chlorine at 5-10 PPM. The filter should operate continuously, to maintain good water circulation in the deep end. Poor circulation will only add to the problem. If you would like to improve the circulation dramatically, adding "The Pool Circulator" to each return will do just that. It is simple to install.  This combination of steps is necessary, in order to control this problem. After the problem is eliminated, resume normal chlorination and filtration. Restore the pH to the optimum range. To help avoid a recurrence, add a weekly dose of the Polymer Algaecide, as directed on the label. I hope that these instructions will prove effective. Enjoy the summer. At least what is left of it.

Sincerely. Alan Schuster, 7/23/2020

Black Algae?  Maybe Yes or Maybe No?

Hello Alan, I have a pool that is about 20 years old, with the original plaster. I am noticing black spots and have been led to believe that this happens with plaster this old. I am under the impression that the only thing to do is drain, acid wash and replaster the pool. Is this true about old plaster actually encouraging black algae growth?  What advice can you offer? Thanks for your help.

Tony B., 1/26/2017

The black spots could be black algae or it could be a problem with the pool finish. A 20-year old plaster finish would be quite susceptible to many staining problems. A rough, badly etched surface can have nooks and crannies that can facilitate algae growth. With the information provid
Ultra Poly One Coat is a Hybrid-Epoxy Coating for refinishing plastered or fiberglass pool, spas or fountains.ed, I can't tell which is more likely. Try this. Place a 3" chlorine tablet on a spot and leave in place overnight. If the stain is removed, it is black algae in all probability. If the stain is not removed, it is either a metals problem or a pool finish problem that will probably require a refinishing. Try this. Place 1/2 pound of pH reducer powder in a white sock and drop onto a stain. Leave in place for 15 minutes. If the stain is removed, the problem is definitely metals: iron, copper, manganese, etc. If not, try placing a few vitamin C tablets on a stain, with the filter off. Leave in place until dissolved. If this does not work, that leaves a pool finish problem are the most likely cause. Realistically, I would not invest a lot of time and money on this problem, given the fact that the pool surface is 20 years old and well past its expected life. That is unless the chlorine tablet, acid or vitamin C worked. If not, it is not unusual to have such problems, as the plaster wears thin.  You have the option of having the pool replastered or painting.  Ultra Poly One Coat is a hybrid-epoxy coating, that is used in pools and water parks, all across the country. It requires easy preparation, with NO acid wash required.  I hope that I have been helpful.

Sincerely. Alan Schuster, 1/26/2017

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Black (Algae) Pool?

Alan, please help. We have a big 24,000 gallon above ground pool. We tried to take the water out (thank God we couldn't) and clean it. Well it is now only about 6 inches below the skimmer, and we need to get it ready for swimming and it has turned BLACK. What and how much algaecide should we use to get it to a point were we can start chlorinating it again. We had the chlorine levels really low and the pump off for the winter. Please help us.

Donna and Michael, Deltona, FL, 2/23/2016

In your e-mail subject line, you used the phrase "black algae." However, from the content of your letter, it is apparent that this may not be the case. Clearly, you need to start by getting the Free Chlorine elevated, as soon as possible. Add 5 pounds
of a quick-dissolving shock or 5 gallons of liquid chlorine. Keep the filter running and make sure that the pH is 7.2-7.6. Retest the Free Chlorine, after a few hours and repeat this dosage, if the Free Chlorine level is below 1-3 PPM. Keep adding chlorine until the Free Chlorine is at least 1-3 PPM, after an overnight period, or the water is noticeably improved. The water may remain cloudy, as the algae is decomposed.  If at this point, you see black deposits on the walls, it is possible that you do have black algae. Treating black algae will require a regimen as follows: boost the Free Chlorine to 10 PPM, add an initial dose of a "quat" algaecide to act as a wetting agent, add an initial of a polymer algaecide, lower the pH to 7.2, redirect the water flow to send more water towards the affected areas and use a brush to scrub the deposits. Once the problem is solved, resume normal chlorination and add a weekly dose of the polymer algaecide. I hope that this information proves helpful.

Sincerely. Alan Schuster, 2/23/2016

Not Quite Black Algae?

My pool developed a black discoloration about 3 feet in diameter on the bottom. It is a 24' round above ground pool. It happened one day after I added a couple of bags of shock. I have been treating it with a black algaecide and shock for more than a week and there is no improvement. I did some browsing through the archives and I thought that I would give the acid powder in a sock a try. I shut the filter off and dropped the sock into the middle of the discoloration. Within minutes, I could see a difference. As the acid was dissolving, it was spreading along the bottom and the stain was disappearing. Soon it was all gone. Can you explain what happened? Thank you!

Stan, Staten Island, NY, 7/23/2009

The one thing that we do know is that it was not black algae. Not all dark discolorations are black algae. Algae is less likely to
METALTRAP Filters remove iron, copper and manganese. appear after a shock treatment. It seems certain that the problem was caused by the presence of some heavy metals: iron, manganese, copper, etc. When you added the shock, the chlorine content rose and the pH changed and that set some precipitation into motion. The fact that the acid so easily removed the discoloration, confirms that the problem was mineral and not algae. You have two more things to do, in case you have not already done them. Add a dose of a quality Mineral Treatment, such as phosphate-free, Liquid METALTRAP, to help complex the minerals that you just dissolved. Bring in a water sample for heavy metal analysis. This may help confirm the problem. Thereafter, use a METALTRAP Filter to remove any heavy metals, prior to the addition of any makeup water. Clearly, things are looking better for you. Enjoy the summer and I'm glad the advice worked so well.

Sincerely. Alan Schuster, 7/23/2009

How to get better control of pool algae problems.

Use a Salt Chlorine Generator for more consistent control of the chlorine level.
Removal of phosphates helps control algae growth and reduces chlorine usage.
A smoother surface, with fewer surface defects, helps eliminates algae growing spots.
The Pool Circulator improves circulation and eliminates dead zones . . . that promote algae growth!!!
Proper water chemistry helps promote sanitizer effectiveness.
MegaChlor salt chlorine generator, for spas and swim spas. TotalTrap Pool Refresh removes phosphates and heavy metals Ultra Poly One Coat is a Hybrid-Epoxy Coating for refinishing plastered or fiberglass pool, spas or fountains. The Pool Circulator is a replacement return jet fitting, that dramatically improves circulation. #2086 ColorQ 2X - 2nd Generation Pool/Spa Tester
A no-installation needed, full-featured Salt Chlorine Generator, for all types of pools. Pool Refresh is a 2-part system, that removes phosphates and helps promote better algae control. A Water Sweeper Brooms helps keep the pool surrounding debris-free and out of the pool. The Pool Circulator eliminates dead zones, improving sanitizer action. Easy installation. ColorQ 2X Digital Water Analyzers eliminate all the color-matching and guesswork.
Click on any image for complete product and ordering information.

Black Algae: Is It Or Isn't It?

We have an above ground 30' round pool which is about 5 years old. The first 2 years, we had no problems with clarity and no problems with water condition. In the 3rd year, we noticed a dark stain on the bottom which runs about 3-4 feet in length about 1' wide in the circular shape of the pool. We've been told repeatedly that we are dealing with black algae. We have gone to just about every pool shop in the area and have tried multitudes of treatment options, including mustard & copper products, while adding the scrubbing morning, noon, night routine. While some of the treatments worked, they only worked temporarily…only to wake up one morning and find the same spot re-appearing. (Perhaps they should call this organism magic algae.) Not only has it been an investment to make our pool clear and stain-free, we have worked very hard to maintain its condition. Since we live in New England, where the pool season is only 3-4 months long, at best, we and our kids enjoy our pool. As we have spent hundreds of dollars over the years on chemicals that we were promised would solve the problem, and have spent plenty of hours cleaning our pool rather than swimming, we are contemplating replacing our liner completely. Do you think this is a radical remedy or do you have one last solution for us? Thanks for any advice you may have to offer. Regards from New England

Bridgitte T., New England, 5/3/2009

MetalTrap Stain Reversal Kit, for pools and spas.
My guess it that it is not "black algae." You apparently have tried to treat this as algae, without success. The stain is probably the result of minerals
such as: copper, iron and/or manganese. A water analysis should help to confirm this. Let's try this! Shut off the filter. Put 1/2 pound of pH reducer in a white sock and drop onto a stained area. Leave in place for 5-10 minutes. Move around with a vacuum pool, afterwards. If improvement is seen, it is positive confirmation that the problem is mineral, most likely due to iron or copper.  A MetalTrap Stain Reversal Kit contains everything you need to remove the stains, eliminate the metals from the pool water and help prevent a recurrence, after the chlorine level is restored.  Good luck and enjoy the season.

Sincerely. Alan Schuster, 5/3/2009

Black Pool Algae Or Not?

Large pool facility, i.e. water park, has an accumulation of slippery blackish-green stuff on a wooden stair bridge walkway. There is a large pool that runs under the bridge of stairs. The facility operations manager says the blackish-green stuff on the stairs is black algae. This area is an open area and traveled over frequently, meaning that it's kept sufficiently watered from pedestrians walking over it. Is it possible for algae, any type of algae to grow on stairs? Also, would the same techniques to get rid of it be that of getting rid of it from a pool? Thanks so much for your help!

Melanie C., 5/20/2016

Unless you're a microbiologist, it is difficult to put labels on microorganisms. To me it sounds like mildew or fungus. Could be it algae or something else? Probably. The easiest way to eliminate the problem is with power washing and/or spraying the area with liquid chlorine. It's not permanent and it will come back. Inasmuch as people walk on the area, I would not suggest applying algaecides to the surface, so as to avoid creating a slippery surface. This is very common problem in Florida and other damp, humid and warm locations: on sidewalks, patios and roofing tiles. I hope the advice helps.

Sincerely. Alan Schuster, 5/20/2016

Out Of Reach?

Your web site has been extremely useful. I have an Continuous Swim Pool and initially used the mineral purifier system with low level chlorine levels. After a few months, I noted a sticky dark area on the vinyl floor near one of the benches. where the return water goes for the swim current. I am sure it is black algae and it developed under one of the benches because of poor circulation and low chlorine levels.  The swim current is on no more than 30 minutes or so a day and the area under the bench is isolated from the normal pool circulation.  There is no way to scrub that area unless I partially drain the pool and dismantle the whole swim current propulsion assembly and benches.  On the other hand, I have improved the exposed area with treatment using a quat, polyquat, and adding some copper. I am using bromine now with non-chlorine and chlorine granular shock (putting some granules through a crack between the bench and wall). I stopped the mineral purifier. I am scrubbing the exposed area. Is it possible to eradicate black algae without scrubbing behind the benches?  Can I expect to at least control it doing what I am doing? Dismantling the system would be a major undertaking especially if I could not eradicate the black algae completely anyway or if it came right back. I would appreciate your thoughts.

Sean H., Jacksonville, NC, 3/18/2007

Assuming that it is black algae, you seem to have followed the regimen that I normally recommend. However, the use of the copp
er algaecide was not something that I would have suggested, under these circumstances. The mineral sanitizer was already contributing copper to the water. The problem in dealing with some types of algae is that they form a water repellent film on their surface which can act as a chemical barrier.  Using both a polymer and quat algaecide might provide better results, given the bromine is destroyed by the SUN's UV rays.  Try and add the bromine, after sunset. You have correctly concluded, the combination of inaccessibility and poor circulation is not helping the situation. You can dramatically improve circulation an reap the benefits, by installing The Circulator, in each return jet fitting.  Temporarily lowering the pH to 7.0 and boosting the bromine level to 10 PPM and keeping it there for a few days might help. Another viable option would be to use a power washer to help dislodge the algae, allowing the chemicals to better act in destroying the growth. You should be able to use the power washer under water. Once you eliminate the problem, I suggest that you consider adding an ozonator, as backup to the bromine. Good luck and I hope that I have been of help. Let me know how it turns out!

Sincerely. Alan Schuster, 3/18/2007

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