Blue-Green 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!

The most common algae found in swimming pools is the blue-green variety. It is virtually always present, in some minuscule amount, somewhere in the pool environs. As soon as the pool sanitizer level drops too low and the water conditions are right, this type of algae is likely to start blooming and cause a green water problem. Having an effective algaecide, already in the pool water, will keep the algae from blooming quickly and might avoid a visibly serious problem. Blue-green algae responds well to shock treatment. The cloudiness that can result, after the shock treatment, is most likely due to dead algae. The addition of a liquid clarifier can help the filter remove the dead algae and organic debris more quickly. 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 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.

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.

► Keeps Coming Back?

Every year I seem to get problems with green algae growing, in the corners of the deep end.  My chlorine levels test out OK and the water chemistry is good.  I don't like to use algaecides, because of past foaming problems.  Any good suggestions?  Thanks

Paul L., Zwolle, LA, 2/25/2019

Your free chlorine level and overall chemistry might be alright, but it would seem that the water circulation is not up to par.  Algae will start to grow first, in areas that are stagnant or with poor circulation.  Pointing return jets towards the problem areas will help, but you'll lose skimming action.  Installing The Pool Circulator, which is as simple as replacing the existing return fittings, will create a spiraling return flow.  This will vastly improve circulation and eliminate the dead zones, that promote algae growth.  Adding a salt chlorine generator will help you maintain a more consistent free chlorine level, which might have to be maintained at 2-4 PPM, as opposed to 1-3 PPM, to better control this problem.  Adding a robotic pool cleaner will improve circulation, by traversing the whole pool and acting as a moving main drain. The important thing is to improve circulation, so that the sanitizers can be delivered throughout the pool.

If this website was helpful, in answering your question, please consider joining our E-Letter Mailing List.  You'll receive 1-2 E-Letters a month, with helpful information, new product updates, suggestions and sale announcements. I hope that this recommendation works out for you.

Sincerely.  Alan Schuster, 2/25/2019
 

► Green Pool Water?

Do I want to add an algaecide while I'm shocking the pool to get rid of green water? I have received differing opinions.

Stephen, 3/19/2015

Sounds like a pool opening? The problem of green pool water is usually cause by the growth of algae. If this is the case, make sure that you vacuum out any debris, as soon as possible. It will help reduce the amount of shock needed to clear up the water. Always allow each chemical to dissolve in turn. Never mix different chemicals together. Algaecides are commonly added before or after shocking. To get rid of the algae it is important to maintain a Free Chlorine reading of at least 1-3 PPM, after an overnight period. It may be necessary to add more than 1 dose of shocking, depending upon the condition of the pool. Test the water a few hours after addition and add more shock, if required.  The best product would be a 60% polymer algaecide, which is non-staining and non-foaming.  I hope that I have been helpful.

Sincerely. Alan Schuster, 3/20/2015

► Green Pool Algae?

This is our first year of pool ownership. The pool is an 18' round, above ground pool. We seem to have developed some algae on one side of the pool and the water has a green tint. I use a chlorine floating feeder, with 3" tablets and have been very careful about the pH, alkalinity and stabilizer. I clean the cartridge filter weekly and run it for 8 hours a day. What went wrong? Please help because I won't let the kids in the water.

Barbara L., E. Northport, NY, 7/14/2019

You have the most common type of swimming pool algae: the type that causes green water problems. It does sound like you are taking good care of the pool. Still, bad things can happen to good pools! This is really not that bad, just an inconvenience. Algae is always present in swimming pool water. If the conditions are right, it will begin to grow, resulting in visible signs of algae or problems with green water. That means, if the chlorine level has bottomed out because there wasn't enough in the water or the demands of the bathers was just too high, algae can start to grow. From your letter, I cannot determine the actual cause, but I can make an educated, insightful guess. The chlorine floater may not always be able to supply all the chlorine that the pool requires, because the tablets are slow dissolving. Floaters are popular with above ground pools, but are not necessarily the best way to add chlorine. It may be necessary to supplement the chlorine floater, by the addition of a quick dissolving product: sodium dichlor, liquid chlorine, lithium hypochlorite, non-chlorine, shock, etc. Or, you might consider a salt chlorine generator, instead of the floater or additions of shock. It will provide better control  and can be used to give the chlorine a quick boost during those periods of high bather usage: like when all the kids are in the water for hours at a time. You are in control, by selecting the level of chlorine output,  Test the water for Free Chlorine, several times a day, to gauge the requirements of your pool, during these periods of peak activity. I suggest that you use an algaecide to help prevent the growth of algae. To get rid of the algae, add 2 pounds of shock, per 5000 gallons of water. Circulate continuously. Test the water for Free Chlorine and keep the level at 2-3 PPM or higher, until the algae is gone. Thereafter, resume normal chlorination and filtration. Because the algae formed on one side of the pool, try and direct more water to that area. Swimming can resume, when normal chlorine levels return.  Most above ground pools do not have main drains and this can result in poor water circulation on the bottom and in the corners. Adding a robotic pool cleaner will not only help keep the pool clean, it will improve water circulation and reduce the possibility of algae growth. I hope that this information will help to get the kids back into the water. Glad to be of assistance.

Sincerely. Alan Schuster, 7/14/2019


► Blue-Green Pool Algae?

I have a problem with what looks like a blue-green algae bloom in my pool. It is a vinyl, inground. I use a built-in chlorinator, with big tablets. I have had algae and green water problems before - I think it is because of the kids and you know what. Shocking usually does the trick. I haven't been using algaecides. Should I and which would you suggest? Thanks much.

Brenda R., Columbus, OH, 7/11/2020

In dealing with the blue-green algae, almost any algaecide can be used. It is not resistant to treatment. My choice would be a Chelated Copper Algaecide: it is effective, cost-efficient, does not foam and because it is chelated (stabilized) it should not cause staining. Just make sure that you use it as directed by the label, don't add more, as most people do. Tell the kids that you bought a new product that detects the presence of urine - only problem is that the product doesn't exist. Algaecide alone will not be enough, to prevent a recurrence, of the problem.  You need better and more reliable sanitation and a salt chlorine generator is just about the best way to accomplish that.  You can dial up the chlorine output, based on usage and the seasons.  You will not longer have to store, measure or handle chlorine products and you'll avoid the relentless buildup of cyanuric acid.  Hope that I've helped. Enjoy the summer.

Sincerely. Alan Schuster, 7/11/2020
 

► Green Algae In The Pool Filter?

Alan, great web site! Keep up the good work. My problem is with recurring green algae in my filter. I have a 14,000 gallon above ground, sand filter, a timer, a chlorinator and very good city supplied water. I had this problem at the end of last season, and it came back mid-season this year requiring me to change the sand in the filter it was so bad. The water prior to the problem was perfect, and when I open the pool it's always crystal clear. I routinely check pH, chlorine, and alkalinity. My chlorine is easy, the chlorinate and timer keep that right where I want it. The pH tends to drop after rain or heavy use, so I add soda ash when needed to boost it back to 7.4-7.6. Alkalinity usually runs a bit high, up to 180. Normally this is all it takes. Then the water started to slowly get cloudy despite my efforts to keep it clean. Over a period of two weeks it went from just noticeable, to almost cannot see the bottom. Based on an hunch I opened the filter and sure enough the sand had so much green algae that several washings with clean tap wouldn't clean it (in a wheel barrow, stirring with a rake). So I changed out the sand, added shock and algae preventative, and now it's crystal clear. When I back-washed, naturally the sight glass was quite green for about 90 seconds, then went clear. 

My question is why is this happening and how do I prevent it? Should I add shock and /or algaecide down the filter basket as a preventative? I admit I don't shock the water often because my chlorine always looks so good. I think I should shock more often, agreed? Will the algae the filter picked up contaminate the filter again?

Ken J., Trumbull, CT, 7/20/2017

There's nothing in your letter that jumps out and says fix me! There is always algae present in the filter and in the environs of the pool. When conditions are right, the algae will grow. There are two bits of information missing from your letter and that can be the clue to the problem. You did not state how you are testing for chlorine or if you are testing for Free Chlorine. If you are using OTO, you may be giving yourself a false sense of security. OTO measures total chlorine and is not the germicidal form of chlorine. You should be testing for Free Chlorine, which is the active and algae-controlling form of chlorine. Keep the level at 1-3 PPM. The total chlorine reading should not be more that 1 PPM higher. If it is you should shock to break down the combined chlorine. You also failed to mention the chlorine stabilizer level. If your level is higher that 150 PPM, it can reduce the effectiveness of the chlorine. If that is the case, you should exchange a foot or two of water, weekly, until a more suitable level has been reached. The right pool water tester can spare you a lot of problems.  The ColorQ PRO 7 is an all-digital tester than eliminates the guesswork and all the color-matching.  It would be my first choice.  You might want to consider adding algaecide on a regular basis. My first choice would be a polymer formula. It can help with both the algae and the filtration.  Sand filter can be very inefficient.  You can boost efficiency, by replacing the sand with a zeolite sand replacement media.  I hope that I have been helpful. Good luck and enjoy the summer.
 
Sincerely. Alan Schuster, 7/20/2017


► Blue-Green Color?

We have just set up our above ground pool. The top of the water looks clear, but the bottom of the pool looks bluish green in color. It is not sticking to the pool and I have tried to vacuum it out, but of course, it is not working. Is it algae? And how can I get it out and filter through my pump? Also, my husband put some algaecide in it from last year. Could this have caused the problem in the first place?

Vicki N., 5/6/2010

It seems likely that it is algae: our old friend cyanobacteria or blue-green algae. It probably developed over the winter and early spring.  Assuming it is algae, you must shock the pool repeatedly, until you are able to detect 1-3 PPM of Free Chlorine, after an overnight period. That means you should add shock to boost the Free Chlorine level to 5-10 PPM. It may be necessary to add shock more than once: test the water frequently to be sure. The longer this takes - the longer the algae will grow. As the algae is destroyed, the water may cloud up, due to the dead and decomposing algae. Filter continuously and try adding a liquid clarifier, to help improve the water clarity and quality. Make sure that the pH is within the 7.2-7.6 range, in order to maintain chlorine efficiency. Algaecide does not cause algae to grow - it helps control algae. Why the problem developed is probably related to how the pool was closed and the ability of the cover to keep out contamination. I hope that I have been helpful. Enjoy the season.

Sincerely. Alan Schuster, 5/6/2010


► Dead Spot?

I keep getting green algae growing in the same area. I can get rid of it by shocking, but it returns even though my chlorine levels are good. Could it be poor circulation? I have only one return, in a 12 x 24 free form pool. Thanks.

Mike N., Sarasota, FL, 6/1/2007

Dead spots created by poor or inadequate circulation can certainly favor the growth of algae. Poor circulation probably means less chlorine and sets the stage for algae to take hold. If you are operating the filter for long enough periods of time, you might trying redirecting the return flow, so as to send more water towards the affected areas. However, this may create another dead spot. There is a gadget called "The Pool Circulator" that can be easily installed in the return fitting and will improve the circulation up to 15 times. If the algae problem is due to a dead spot, this should solve the problem. I hope that this information will prove helpful.

Sincerely. Alan Schuster, 6/1/2007

► Making Progress?

Dear Alan, as first time home and pool owners your website has been such a blessing! We opened our pool about two weeks ago and it was a smelly swampy disgusting mess. There were several holes in the cover and we have 3 large overgrown maples that dumped all the leaves into the pool all autumn! I had told my husband there is no way I would swim in that water even if it did get cleaned up and that we should just empty it and start again. Well fortunately, I decided to surf the web and see what I could learn. After many websites willing to sell me a book on pool opening and maintenance, I came to your site. Thank you! Thank you! Thank you! 
 
After about 20 or so pounds of shock, continuous filtration and algaecide, I can actually see half way into the pool. It has gone from BLACK water, to a cloudy green (which probably sounds gross but it is such an improvement I am thrilled to see it.) I am confident now that with in a week I will have a crystal clear sparkling pool. I know I am not there yet but I am keeping he chlorine at about 3 and I am going to the store today to purchase the clarifier. Believe it or not I do have a question. I understand from my neighbors that the former owners swam last summer with green water so I would assume it looked bad before they closed. Is there something I should look out for as far as that is concerned? Could it mean there was trouble with the filter? Or is that from poor maintenance? Besides the large filter that is connected to the pool there is also a smaller filter that was probably used but if it were green I would guess it doesn't work very well. Can you recommend anything with such small amounts of information? The pool is a 24ft round above ground that I believe has a vinyl liner. Thank you again for your help. I couldn't have gotten this far with it.

Jennifer V., Valparaiso, IN, 6/5/2008

Thanks for the fan mail! It does sound like you are making progress. Getting a bit more aggressive with the chlorine should help speed things up. Try and boost the Free Chlorine to 5-10 PPM and keep the pH near 7.2. Adding algaecide can help deal with the algae. Your pool does not need 2 filters: just one that works properly and is run for a long enough period. Filtration alone cannot possibly control algae in a swimming pool. Swimming in a pool that is not sanitized properly brings certain risks. Algae may seen harmless, but infectious bacteria are not.  You want to make clear water with a proper chlorine level for maximum pool enjoyment and safety. The presence of phosphates and nitrates can accelerate algae growth and increase the chlorine demand. Some dealers can perform these tests and it can be worthwhile.  I hope that I have been helpful and that you have a good season.

Sincerely. Alan Schuster, 6/5/2008


► Not Working Out?

My pool goes green by five days after the pool guy has been there. My pool guy who has never been at my pool for more than sixty consecutive minutes, says the water's bad. He's been dropping two gallons of chlorine and a week later its green. He also suggests a new larger filter. Does water go bad? We fill up constantly, due to evaporation and we get some rain... flush is okay with me but not unless we have to. And black algae comes around soon and we have to shock, which doesn't really work.

Eric P., 8/9/2014

You probably don't need a new filter, just a new way to add chlorine. Adding two gallons on day one, can work for some people, but it isn't working for you. You need to add chlorine throughout the week and do some testing, as well.  Either get yourself an inline chlorinator or, better still, a salt chlorine generator. It is chlorine without all the negatives. No chlorine odor, no handling, storage or buying chlorine and much better water quality. You'll have to buy some salt - common, non-iodized food grade or water softener grade - inexpensive! Thereafter, you need to add more salt only to replace that lost through pump out, splash out, backwashing or overflow. With a salt chlorine generator, all you need to do is add an initial dose to stabilizer to bring the level up to 40-60 PPM. Thereafter, more stabilizer is needed only to replace that lost through pump outs, backwashing and splash out. The overall water chemistry should be maintained in the usual manner. Because salt chlorinators destroy chloramines so effectively, you will find the swimming conditions more pleasant and easier to maintain. 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.  I hope that this information will prove to be useful.

Sincerely. Alan Schuster, 8/10/2014


► Algae And Bubbles?

Alan, I have had a problem for the last year with my inground pool that is driving me nuts and making me want to fill in my pool! There are bubbles coming out of the returns of my pool. These bubbles adhere to the sides of the pool which then encourage algae growth! I am opening my pool now and the bubble problem still exists as well as the fact that the pool was so full of algae when I took the cover off, 4 frogs had made nests in the pool water! Help!

Becky, 5/16/2009

It would not be uncommon for this problem to occur, after the springtime pool opening. I suspect that there is an air leak in the return line - somewhere between the filter and the return ports, as this would cause air bubbles, in the return flow.  Check all the connections. You are associating the bubbles with the algae problem and that is not likely to be the case. The algae problem is directly related to the lack of chlorine at the opening of the pool. You have to shock treat, until you establish a free chlorine reading of 1-3 PPM that lasts through the night. Add a pound or two per 10,000 gallons, every few hours, until the water improves or the free chlorine level reaches 5-10 PPM. If you want to use an algaecide avoid quats. Use either a polymer or copper algaecide. It will probably take a lot more shock or chlorine than you expect, given the poor condition of the water. Be prepared - don't drag it out, as it will only require more chlorine. If you plan ahead, the benefits of a salt chlorine generator will become apparent:  more control, better result and no more handling measuring and storing of chlorine products.  Good luck and I hope that the information is helpful.

Sincerely. Alan Schuster, 5/17/2009


► Copper Sulfate?

I was told that the above chemical was all you needed for your swimming pool? They suggested one tablespoon every two weeks. My pool hold 13,000 gallons of water. So far I have had no problems. Your comments will be appreciated. Thanks.

Ron, 8/8/2008

No company in the industry recommends copper sulfate. NONE! At least not in the form of 100% copper sulfate. It is great in reservoirs, but in a swimming pool it can lead to staining, discoloration, green hair and fingernails and more. At best it will help control algae. But, it is not a pool water sanitizer and will not control bacteria. If you want to use copper, as an algaecide, there are plenty of products available.  None are copper sulfate. I hope that I have been helpful.

Sincerely. Alan Schuster, 8/8/2008


► I've Shocked and Shocked?

My pool developed algae because it was neglected for a week or two. I brought a water sample into a local pool store and the only thing wrong was that there was no chlorine. They gave me 4 pounds of shock and a bottle of algaecide. I was told to add 2 pounds of shock and some algaecide. The next day I was supposed to add the other two pounds. I did all that and there is still no chlorine reading and the pool is cloudy. There is still some algae left. I went back and bought another two pounds and still no chlorine. It has been 4 days now and the water is cloudy and there is no chlorine. The algae seems to be gone. I have backwashed the sand filter 2 or 3 times. What should I try next?

Jim G., 8/2/2006

From your description, it does seem that you have made progress. You didn't tell me how big the pool is, so I can't relate to the amount of shock added. However, it is clear that you did not add enough! The addition of two pound increments was probably reasonable for your size pool. You must keep adding shock, at the same rate, until a 1-3 PPM Free Chlorine Test reading is established and lasts through the night. It is important to do this quickly. The longer you drag it out - the more the algae will grow, increasing the total amount of shock required. Once a stable Free Chlorine reading is achieved, normal chlorination should be resumed. The presence of phosphates and nitrates can accelerate algae growth and increase the chlorine demand. Many dealers perform these tests and it can be important. During this period operate the filter continuously. The practice of frequent backwashing is wrong. Sand filters should not be backwashed daily: usually only when the pressure is too high or at periodic intervals. Frequent backwashing lowers the filter efficiency and could be responsible for the cloudy water. The dead algae and organic debris could be passing through the filter. Try adding a liquid clarifier, to improve filter efficiency and help remove the suspended particles that cause cloudy, murky water. Have you considered adding The Pool Circulator? Not only will this gadget save you time and effort, but in addition to getting clearer water, you will improve the circulation on the bottom and reduce the possibility of algae growth.  It simply replaces the standard return jet eyeball fitting and creates a spiraling return flow, that reaches throughout the pool.  Enjoy the summer. I hope that I have been helpful.

Sincerely. Alan Schuster, 8/2/2006

► 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 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.  I hope the advice helps.

Sincerely. Alan Schuster, 5/20/2016

Visit The Website Stores . . . for better informed shopping!!!

The Website Store New Products Store Test Equipment Store What's On Sale Spa Products Store Products and Info Products and Info Products and Info Products and Info Products and Info Shipping is FREE* . . . within the Continental U.S. A $9.99 handling charge will apply to Continental U.S. 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.  International and orders outside of Continental U.S. - see comments on the ordering pages. Major Credit Cards and PayPal are accepted.