Swimming Pool Algae - Identify, Treat, Prevent

Knowing which kind of algae has taken up residence in your pool is the first step. There are literally thousands of varieties of pool algae - but for our purposes we classify pool algae according to color. Most pool algaecides will have an effect on most types of pool algae, but some treatments are specifically manufactured to fight specific strains of pool algae.

Yellow Pool Algae

Also called Mustard Algae, it has a greenish-yellow color, and attaches itself to crevices and prefers shady spots in the pool. Yellow algae tends to brush off easily, unless your pool surface is very pitted and etched. Can effectively hide itself in locations deep in the skimmer, or in your filter, away from direct contact with your chemicals and pool brush.

Green Pool Algae

Green pool algae can be wall clinging, or it can be free floating. At times, it can take on a greenish-blue hue. The most common type of pool algae, green algae is also one of the fastest growing. Left untreated on a hot weekend, the bloom can rapidly take over the pool. It can be clear and almost neon green at times - or it can present itself in a cloudy, dark green color.

Black Pool Algae

The most insidious strain of pool algae, Black algae puts down roots and intends to stay awhile. Left untreated and black algae becomes almost impossible to control. Large heads or "caps" on this variety protect the organism which has made a home in tiny surface irregularities. The heads can be as small as a pencil dot, or as large as a dime. Prefers shady and out of the way locations in your pool.

Pink Pool Algae

Pink algae is not really algae at all, but a strain of bacteria. A form of red algae, it can be introduced to your pool from the ocean, or come in on a strong breeze. It rarely blooms over the entire pool, but will show itself in small etched areas, around the tile or skimmers on plaster pools. In vinyl pools, you can spot pink algae hiding around the edges of plastic faceplates, step sections and underneath ladder treads.

Pool Algae Treatment

Yellow Pool Algae

The preferred method to fight Mustard algae is to use a Chlorine Enhancer, such as Yellow Out. Following the instructions, you will raise the pH to 7.8, raise chlorine to 2-3 ppm, and add Yellow Out followed by a measured dose of granular pool shock. Brush the pool thoroughly and then allow the algae to settle to the bottom where you can vacuum the dead cells and shock dust into the filter. If you have a multiport valve, vacuum the debris out of the system by using the waste setting.

green algae pool

Now, the next steps are very important, if you want to eradicate yellow algae from your pool. Wash all swimsuits on a long, very hot cycle, with bleach. Place skimmer nets, brushes, vacuum heads, etc. in a trash can with a strong bleach solution, (you can use dissolved pool shock) or replace them if they are excessively worn. Do the same with any pool floats or pool toys that have come in contact with the water, during the algae bloom.

Finally, and especially for large blooms of yellow algae, replace or thoroughly clean filter media. DE grids and filter cartridges can be removed and soaked in a large can full of a bleach solution, or replace them if they have some age to them. If you have a sand filter, you should change the filter sand.

Green Pool Algae

Swimming pools with regular green pool algae blooms should invest in a bottle of Phosphate Remover. Phosphates are one of green algae's main food sources, and like unwanted guests, they usually leave when the food runs out.

For sporadic blooms, or spring-time opening algae, green algae can be one of the easiest forms of algae to eradicate. If your pool is very polluted with green algae however, to the point where you can't see the floor in the shallow end - consider diluting the pool water. In extreme cases such as this, you will have an easier, cheaper cure by draining half of the pool water and refilling. This is especially true for those pools that have marginal filtration - undersized or less than effective pool filters.

In most cases however, you will be able to administer a killing dose of chlorine to knock green algae into submission. Using a chlorine enhancer like Swamp Treat will accelerate your chlorine's killing power and make the job easier. In either case, first balance the water chemistry and adjust your pH level to 7.2. Chlorine has much greater efficacy or power at a low pH, and algae blooms much more easily at a higher pH level.

The biggest mistake people make when adding chlorine to the pool when trying to kill algae is simply not adding enough chlorine. If the pool is 30,000 gallons and you add 3 pounds of chlorine in an attempt to kill algae, then you just wasted all of that shock. The pool will still be green, and you’ll be out 3 pounds of chlorine. Always be sure to hit the pool hard enough with chlorine when trying to kill algae. If the shock is only on the wall, in isolated areas, pour the granules down the wall - in that area.

If the algae is blooming all over the pool, looking like pea soup - then broadcast your shock over the water, evenly distributing the granules. Remember to lower the pH first, as chlorine is much more active at a pH range of 7.2 - 7.4. Keep shocking the water until the pool turns a blue/gray color. (remove solar blankets and pool cleaners to protect them from the high chlorine). Then Brush! The other mistake people make is they don't brush thoroughly enough. It can be difficult, tiring - and you need a good brush. Weekly brushing can help prevent green algae from returning, by keeping the small pores of your plaster clean, and allowing your algae fighting chemicals to make close contact in these areas.

Black Pool Algae

Proper water balance is important for all algae treatments, but even more so for black algae. Especially your pool's pH level - try to keep it on the low side, in the 7.2 range, especially during treatment. Keep enough pH down on hand to maintain, if your pH level tends to rise.

For plastered pools, you will need to use a steel-bristled pool brush to rip into the heads of black algae. These heads have a protective coating that blocks your pool chemicals from entering the organism. Another method that tends to be more effective is to get in the pool with a mask and a big breath of air. Using a pumice stone, putty knife or a chlorine tablet, scrape the heads off of the wall.

Once the heads have been knocked off, shock the pool with a double-dose of pool shock, or 1lb per 5000 gallons. Using a chlorine enhancer such as Yellow Out or Swamp Treat will ensure greater success. After shocking the pool, carefully vacuum up the algae debris and shock dust. If you have a multiport valve, fill the pool level up high, and vacuum to waste.

After shock treatment, follow the advice above for yellow algae. Wash all swimsuits on a long, very hot cycle, with bleach. Place skimmer nets, brushes, vacuum heads, etc. in a trash can with a strong bleach solution, or replace them if they are excessively worn. Do the same with any pool floats or pool toys that have come in contact with the water, during the algae bloom.

Pull out ladders, and look underneath the ladder treads. Soak these in a chlorine solution and pressure wash them to remove all trace. Likewise with your pool light - another area that algae can hide, behind the light. Pull the light out and place it on deck for a good scrubbing, then take a look inside the light niche for any evidence of algae. Remove any found with vigorous scrubbing.

Finally, replace or thoroughly clean filter media. DE grids and filter cartridges can be removed and soaked in a large can full of a bleach solution, or replace them if they have some age to them. If you have a sand filter, you should change the filter sand, especially the top few inches of sand.

After a few days, once the chlorine level has dropped below 3.0 ppm, add an initial dose of Black Algaecide to the pool. This is a copper-based algaecide which is very effective at controlling black algae. Colloidal silver algaecides can also be an effective treatment for black algae. Either of these metal algaecides are a complementary and necessary step - after you have delivered a good dose of chlorine directly into the cell membranes.

For extreme conditions - when black algae has taken over and your pool looks like chocolate chip ice cream, a drain and clean is indicated. Carefully and properly drain the pool, and then pressure wash the pool, followed by a chlorine wash. Use regular Clorox, or liquid pool chlorine, if available in your area. If you want to take the extra step, follow up with an acid wash - but be sure that all of the chlorine is thoroughly rinsed and pumped out of the pool. Chlorine and Acid do not mix well! It creates something like a Mustard Gas.

Pink Pool Algae

Pink algae is usually very localized, and as such, tends to be easier to treat. As with all of our algae treatments listed above, the first step is to shock the pool hard. You can use a chlorine enhancer product to assist. Raise the chlorine level to at least 10 ppm, and follow up with vigorous brushing. If your pool brush is too cumbersome, try using a kitchen scrubber tool or a small hand held wire brush.

After the chlorine level has subsided below 3.0ppm, you can add a Polymer algaecide, one labeled with the number 60, which indicates a 60% polymer solution. "50" is a different type, a family known as Quats, which are not effective at fighting pink algae. You can also use a bacteristat such as colloidal silver, or Silver Algaecide - which can be effective at controlling and prevent new outbreaks.

If you've read this far - congratulations, here's a bonus tip. Never shock the pool and add algaecide at the same time. In most cases, very high levels of chlorine will interfere with the effectiveness or your algaecide. For polymer algaecides, high chlorine levels will break apart the polymer chains and render your expensive bottle of algaecide 60 - useless.

Algae Attack Value Packs

algae attack pack

New! For those that want to avoid confusion in their selection of algae fighting chemicals, we have put together three Algae Attack Packs, that will treat Black, Green or Yellow algae in pools up to 30,000 gallons. No more uncertainty - just buy the Algae Attack Pack that matches your algae color. Our in-house pool experts have put together these powerful packs of algae fighting chemicals to kill specific algae types, and prevent their return.

Pool Algae Prevention

  1. Maintain proper water balance: pH, Alkalinity, Calcium and Cyanuric Acid levels.
  2. Use a Phosphate Remover chemical to remove the main food source for algae.
  3. Add Phosphate Removers if fertilizers accidentally blow into the pool.
  4. Maintain your chlorine level in the pool at 1.0-2.0 ppm and shock regularly.
  5. Add a regular dose of a quality algaecide. The cheap stuff is less effective.

A word on Filtration. If your filter is not running long enough per day, or is undersized, old or ineffective, this will exacerbate an algae problem. Effective filtration is operating the pump long enough so that all of the water in the pool is filtered at least once per day. If you try to save too much money on your electrical bill, you may end up spending these savings on a chemical bill.

A word on Circulation. Some pools have poor circulation, or areas of the pool that don't receive much action. If you notice a cloudy deep end, for example, or an area that never looks as clean as other parts of the pool, you may have circulation issues. You may be able to correct by adjusting return fittings to create a different flow pattern. Adding an automatic pool cleaner is a good solution for older pools that have undersized plumbing. Most newer pools do not have circulation problems.

A word on Sanitation. As mentioned above, item #4 is important to keep algae at bay. Algae cells are always in the pool, just waiting for you to slip-up, and forget to add chlorine for a few days. Having an algaecide residual in the pool water will help prevent the bloom, but you will have a much better chance to prevent a re-occurrence by keeping your chlorine levels maintained.

I hope that this information on pool algae has been helpful to you. I've battled a few blooms in my day, and it's never fun. In hindsight, it's always best to Prevent, rather than Treat. Be vigilant this year, and if you reach your wit's end - give us a call, we'd love to help.

In The Swim makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and In The Swim assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. In The Swim assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.