. . . a resource for pool and spa help and informed shopping!!!

Your Mobile-Friendly, Safe and Secure, Pool and Spa Connection, since 2002.


Home page icom

  A backyard pool to enjoy all season long.   A backyard spa for pure enjoyment.   Daily Tips - Helpful Info.   Shopping in the website stores.   ColorQ digital water analyzers, for pools and spas.   Big sale in the website store.   help button  



On Sale
  E-Mail A

Black Pool Algae Problems

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

Treating Resistant Pool Algae Problems.


Scroll down to browse through some archived SWIMMING POOL questions and answers.  Please click the Pool Topics 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!

NUVO Ultraviolet Sterizers for Residential Pools. The Circulator boosts circulation and eliminates dead zones.
Model SR salt chlorine generator, for all types of pools. Card On Guard, Solar UV Sanitizer, for pools and spas.
Solar-Breexe Robotic, Solar-Powered Pool Skimmer-cleaner. A Solar-Powered, Pool Purifier and Mineralizer release copper and zinc ions and helps provides sanitation and back up algae protection.  The combination of better sanitation and improved circulation, can be very effective, in controlling algae problems and providing more consistent results. Relaiant salt chlorine generators, 3-models, for all types of pools, up to 40,000 gallons.

Click any image for more product and ordering information.

Free Shipping

* FREE Shipping . . . within the Continental U.S.

* A $9.99 handling charge will apply to Continental U.S. (48 states) Orders, under $75.00.  Orders outside of the Continental U.S. may require some additional charge, based on quantity and destination.
Most products can be shipped World-Wide.

On-line shopping since 2002 - Safe and Secure!!!

How to treat Black Algae and other Resistant types of pool 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.   If cloudy water persists, due to dead algae, the addition a Pool Nano-Stick Clarifier might be a welcome addition.  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.

Sign-Up for our e-mail list, Join our E-Letter Mailing List.
You'll receive 1-3 E-Letters a month, featuring helpful pool and spa advice, new product information and sale announcements.  All we require is your e-mail address and you can opt out anytime you wish.
Your information will never be shared or sold.

▼     Helpful, Problem-Solving Information, in a question and answer format.     ▼

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, 6/12/2018

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 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, 6/13/2018

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, 7/23/2015

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, 7/23/2015

Maybe Yes - Maybe No?

Hello Alan, I am servicing a pool that is about 20 years old, with the original plaster. I am noticing black algae 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? I don't want the customer to blame me for this situation. What advice should I give him? 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 provided, 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, you are certainly not at fault, as it is not an unusual to have such problems as the plaster wears thin. Present this to the customer, as this is the right time to give the pool a fresh new look. More information on treating black algae or metal stains is available on other pages of this website. I hope that I have been helpful.

Sincerely. Alan Schuster, 1/26/2017

Black Something?

Hi, My name is David and I live in Winter Park Florida. I don't have a pool. But, I do have a large concrete patio, that I built on the back of my house, with a large grapefruit tree next to it. I have had a problem with a black sooty substance on the leaves of the grapefruit tree. Now I have a problem with a black substance on the patio. It starts where the patio connects to the concrete blocks of the house and moves out over the patio towards the yard. I have removed it with a pressure cleaner but each year it comes back. My question is, is it black algae and how do I keep it from coming back? Thank you for your time.

David, Winter Park, FL 2/24/2006

There's nothing in your letter that leaves me to suspect that it anything other than the common mildew condition, that is so widespread in Florida. The heat and humidity will do it every time, especially in shaded areas that remain wet. Power washing does work. Liquid chlorine can be used on the area and will generally do a better cleaning job. I don't have any secret tricks to share. Sorry that I couldn't be more helpful.

Sincerely. Alan Schuster, 2/23/2006

big sale graphic

Look at Some of What's On Sale!!!

Limited-Time-Only Savings on interesting products.
Shop with Security and Safety.
Protected on a secure server, with SSL encryption.
ColorQ digital water analyzer. Remote-Controlled Pool Racing Boats NUVO Ultraviolet Sterilizers for residental pools, of all types. Solar-Powered Mineralizer for pools. Remote-Controlled pool surface skimmer.
ColorQ All-Digital
Water Analyzers
Pool Racing Boats
Ultraviolet Above Ground Pool Sterilizer Solar-Powered
Pool Mineralizer
Pool Skimmer
Visit The Website Store for more unique and problem-solving products.

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, 2/23/2010

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
Nano-Stick Clarifiers, forall types of pools and spas. 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. Adding a Nano-Stick Clarifier can help oxidize and eliminate the fine debris, that detracts from optimum water quality and clarity.  The Nano-Stick is simply hung, from a ladder or rail, and lasts for 4-6 months.  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, 2/23/2010

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.
Add an Ultraviolet Sterilizer and you will dramatically reduce chemical usage.
Add a second layer of back-up sanitation, for better algae control.
The Pool Circulator improves circulation and eliminates dead zones . . . that promote algae growth!!!
Proper water chemistry helps promote sanitizer effectiveness.
Model SR salt chlorine generator, for all types of pools. NUVO Ultraviolet Sterilizers for residental pools, of all types. Solar-Powered Mineralizer for pools. The Pool Circulator is a replacement return jet fitting, that dramatically improves circulation. ColorQ digital water analyzer.
A no-installation needed, full-featured Salt Chlorine Generator, for all pool types. An ultraviolet sterilizer reduces algae and bacteria populations, using fewer chemicals. Solar-Powered
Purifier/Mineralizer, uses copper and zinc ions. Treats 32,000 gallons.
The Pool Circulator eliminates dead zones, improving sanitizer action. Easy installation. ColorQ 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

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
Liquid MetalTrap 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. Repeat elsewhere as needed. Add 2-3 doses of a quality mineral remover, such as phosphate-free, Liquid METALTRAP, in order to complex the minerals and avoid a recurrence. If the areas are too large for the sock treatment, try this! Use a stain remover accessory: available at many pool stores. This device will allow you to siphon a solution onto the stains. Make a solution of 1/2 gallon of water, 1 quart of the mineral treatment and 1 quart of muriatic acid. Use a plastic pail, wear eye protection and rubber gloves and add the acid last. after the stains are removed, make sure that you add the 2-3 doses of mineral treatment before adjusting the pH. I'm confident that this will work. If it doesn't, please get back to me and we'll try plan "B." Refer to other areas of the archives for additional information. Good luck and enjoy the season.

Sincerely. Alan Schuster, 5/3/2009

Black Spots?

First of all the basics: 22,000 vinyl line pool with skimmer and main drain, Chlorine treated Sand filtration. The pool gets black mud looking spots.  They can easily be brushed away, but return. I usually use a copper based algaecide. It has given some relief in the past but seems ineffective now. We keep our pool open year round but only swim in summer, we live in Atlanta. Have you got any ideas? Sincerely.

Mike G., Atlanta, GA, 3/10/2007

My best guess is that it is not black algae: this type of algae is very difficult to remove and will not simply brush away. It is possible that copper can cause some black stains, especially if the water is high in calcium hardness, although this is unusual with the proper use of a chelated copper algaecide. I suggest that you discontinue use of the copper algaecide and start using another type, inasmuch as you stated that it seems not be effective. My choice would a polymer algaecide. It is more expensive, but worth the price. Try this on the spots. Put 1/2 pound of pH reducer in an old white sock, drop onto a spot. Leave in place for a few minutes and move around with a vacuum pole. If improvement is seen, this will confirm that it is a mineral problem and not algae. Repeat elsewhere as needed. To help prevent a recurrence of the problem, add a double dose of a quality mineral remover, such as phosphate-free, Liquid METALTRAP.  I hope that I have been helpful.

Sincerely. Alan Schuster, 3/10/2007

It Worked, But What Is It?

After reading about black algae in the topics section, we were convinced that was what we had in our pool. I started the program that you called for and it seems to be working. We have an inground with vinyl liner. After 4 days some of the spots have went away completely, while a large one is fading. I called the pool dealer that installed the pool to get their ideas on how to treat black algae and their answer was something I had never heard of before. They said that if it was black algae that it was under the liner and it (the liner) would have to be removed and chlorine sprayed on the underside. What is your knowledge of this procedure? Thanks for the information on how to wipe out these awful black spots. Still shocking and brushing.
Durwood W., Arkansas, 6/11/2013

The is an occasional problem with a mold or fungus growing on the reverse side of the liner. It is not usually described as being black: a gray shadow is the most common description. In any event, it does not respond to treatment of the water. What you have seems to be responding to the black algae regimen and there is no reason to suspect that it is anything else. Removing the liner, spraying the back and trying to reinstall the liner could prove difficult to futile. Give the chemicals a chance to complete the task. Enjoy the season.

Sincerely. Alan Schuster, 6/11/2013

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

Water Sweeper Broom in action.
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.  A Water Sweeper Broom is a very convenient to water sweep your deck and patio areas.  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
Solar Pur Pool Mineralizerer 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. As 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. Now that you have added bromine, you can no longer use that particular brand of mineral sanitizer, as it will shorten the life of the cartridge. However, you can use a Solar-Powered Dual-Ion Purifier/Mineralizer, with either chlorine or bromine. 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

Return To Top Of Page












Aqualab Systems., Inc. does not make any warranty or representation, either expressed or implied, regarding the accuracy or completeness of the information provided by this website; nor does Aqualab Systems., Inc. assume any liability of any kind whatsoever related to, or resulting from, any use or reliance on this information. The content of this website should not be used, if it is conflict with any applicable federal, state or local regulations or guidelines.

©, 2002-18, A.S., Inc. All rights reserved