What Causes Red Algae in Swimming Pools?

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What causes algae in pool water is often the same thing that causes red, pink, or rust-colored "algae," which is, oddly, not actually algae at all but bacteria. These bacteria are introduced to pools by wind, rain, swimmers, or contaminated pool accessories. Once in the water, the bacteria usually die unless the water is not maintained correctly, in which case they can breed until red or pink slime starts growing along the walls, corners, or other surfaces. Shocking the pool with high doses of disinfectant will kill any remaining bacteria after you brush, vacuum, and backwash the pool. Once the problem is resolved, you can prevent a recurrence through diligent pool maintenance.


What Is Red Algae?

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Despite regularly being referred to as "red algae" or occasionally "pink slime," red, rust-colored, or pink growths on pool surfaces are actually bacterial growths. The bacteria can grow in specks or larger patches anywhere in the pool and can even make the water itself appear reddish.


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Red algae in a pool isn't just unattractive; it also presents a minor health risk since it releases toxins that can irritate the skin or lungs or cause nausea, vomiting, and dizziness.

Sources of Bacteria

Bacteria are all around us, and it's not surprising that they find their way into swimming pools. The primary source of bacteria in pools is the swimmers themselves, who all carry some bacteria into the pool water on their skin.


If your pool chemicals are out of balance, these bacteria grow and multiply rather than die off. They will build up in the pool water and on surfaces like ladders, pool covers, and inflatable pool toys. But even if you keep your pool water impeccably balanced, bacteria can ride into your pool on the wind and rain in amounts that can overwhelm the pool's cleaning system. Y‌ou may also introduce bacteria to the pool when you fill it with your garden hose.‌


Red Bacteria Treatment Supplies

Eliminating red bacteria from your pool requires quite the arsenal, but you should already have most of these products as a pool owner. Here's what you need to tackle an outbreak:

Bleach:‌ To kill bacteria on your pool toys and bathing suits, you'll need liquid chlorine bleach (we recommend good old-fashioned Clorox, which you can grab at Target for less than $10). While bleaching your swimsuit will help kill bacteria, remember that bleach may fade the colors and damage the fabric. As an alternative, vinegar may kill the bacteria on your suit without affecting its appearance.


Cartridge filter cleaner:‌ Constantly filtering out dead bacteria from a red algae infestation can clog your filter quickly, so keep it clean. While you can backwash sand or DE pool filters, cartridge filters need to be cleaned manually, which can be done easily with a multijetted cleaner that attaches to your hose (we recommend this Aquatix Pro tool from Amazon, $20). Expect to pay between $14 and $80 for one of these devices.



Pool chemical test kit:‌ Proper pH and chemical levels are critical for eliminating a red bacteria outbreak. A chemical test kit allows you to monitor these levels so you know what needs to be adjusted and how. Testing kits vary in price based on how much they can monitor. You need a kit that at least measures pH, alkalinity, water hardness, chlorine, and CYA levels for a red algae infestation. We recommend this chemical test kit from Taylor (Amazon, $108), but these usually range between $20 and $150.


pH reducer:‌ To lower the pH level in your pool, you will need either muriatic acid (we recommend this one from Sunnyside Corporation for $29 on Amazon) or dry acid (we recommend a pool and spa balance by Clorox for $9 on Amazon). To avoid the risk of injury, always be cautious when using pool chemicals, particularly muriatic acid. Follow manufacturer directions and always wear proper safety gear while handling it.


Pool brush:‌ To get rid of red algae growing on the pool's surfaces, you'll need a brush. These cost between $10 and $50, depending on the size, quality, and materials.

Pool vacuum:‌ While you can follow up scrubbing with a manual vacuum attachment, an automatic pool cleaner can make things at least a little easier. A manual vacuum costs between $20 and $80, while robotic models are between $150 and $325. We recommend a cordless robotic vacuum by AIPER, which you can get on Amazon for $299.


Pool shock treatment:‌ You'll want to shock your pool at least twice to kill off the bacteria. We recommend using HTH shock treatments, which will run you about $32 on Amazon for a pack of six. Always follow manufacturer directions when shocking a pool to avoid injury and never enter the water until the chlorine levels return to normal.

Chemical filter cleaner:‌ To prevent red algae reinfection, use a chemical filter cleaner near the end of the treatment process regardless of your pool filter type. These vary dramatically in price based on the bottle size and concentration but expect to pay at least $20 a bottle. We recommend HTH's chemical filter cleaner, which you can find on Amazon for $39.



How to Get Rid of Bacteria in Pools

If red bacteria are plaguing your pool, close it off to swimmers until you fix the problem. To eradicate the outbreak, take the following steps:

  1. Take everything out of the pool, including removable ladders. Put on safety gear like goggles before mixing together 1 part bleach with 1 part water in a bucket. Clean everything you've removed from the pool with the bleach solution and a soft scrub brush.

  2. Bleach your swimsuits to sanitize them.

  3. Run the pool filter 24 hours a day, backwashing or cleaning it twice daily until the bacteria are gone. If your cartridge filter has more than 3,000 hours on it, replace it.

  4. Balance your pool's chemical levels:
    — Use muriatic acid or dry acid as needed to bring the pH to between 7.0 and 7.2 (chlorine is more effective at a lower-than-average pH level).
    — Keep the CYA level between 30 and 50 ppm (lower levels of cyanuric acid make it easier to kill bacteria).
    — Adjust the alkalinity to between 80 and 120 ppm.
    — Bring the hardness to between 150 and 250 ppm.

  5. Brush the entire pool surface with a pool brush twice a day until the outbreak is gone. Follow up brushing with vacuuming to help remove any loosened bacteria.

  6. Shock the pool with calcium hypochlorite, increasing the chlorine level to 20 ppm. Test the levels every 10 to 12 hours to ensure the chlorine level is high enough throughout the entire treatment process.

  7. When the chlorine level returns to between 2 and 4 ppm, shock the pool again.

  8. Clean or backwash your filter again to remove the dead and dying bacteria that built up during the treatment process.

  9. Follow your standard filter cleaning with a chemical cleaning to kill off any last surviving bacteria.

  10. Continue filtering, brushing, and vacuuming your pool until the bacteria is gone and the pool chemical levels have returned to normal.

  11. When the chlorine levels lower again and there are no more signs of bacterial growth, retest and balance the chemical levels. If your pH level is too high after adding so much chlorine, reduce the pH level to between 7.2 and 7.8.

Keep an eye on things for the next month. You may need to repeat the process if red algae appear again. Swimmers can return to the pool when the bacteria are gone, and the chemical levels have returned to normal.


If you're wondering how to get rid of algae in a pool at the same time as a red bacteria outbreak, the good news is that the process is largely the same for both, though you may need to shock a pool with chlorine at least four times to kill black algae.

Preventing Red Bacteria

The best way to deal with bacteria problems is to avoid them. The following routine preventive measures may keep the problem from happening:

  • Run your garden hose for a couple of minutes before starting to fill the pool.
  • Clean your pool cover and solar rings twice a year and change your filter material every two to three years as recommended by the manufacturer.
  • Have swimmers take a quick shower before climbing into your pool.
  • Keep a close eye on your pool water's chemical balance.
  • Increase sun exposure to your pool if possible by trimming tree branches in the area.

You'll never be able to eliminate bacteria exposure completely, but you can greatly reduce the amount in your pool with these steps.

Types of Algae

Though red algae in pools isn't technically a true algae, it's not unlikely to think you'll be dealing with an algae issue eventually as a pool owner. There are more than 21,000 known species of algae, but few, if any, of the red varieties are ever found in swimming pools. As with red bacteria, what causes algae in pools is poor maintenance paired with the introduction of algae fragments, cells, and spores that are spread through wind, rain, or contaminated items brought into the water. Algae are plantlike lifeforms that contain chlorophyll and grow through photosynthesis, meaning they breathe in carbon dioxide and exhale oxygen. While there are tens of thousands of types of algae, pool experts typically classify them by their colors, specifically:

Green algae:‌ The most common algae found in pools, green algae reduces water clarity as it clings to the walls in slimy green sheets. These outbreaks are caused by poor filtration and sanitation.


Yellow algae:‌ Sometimes called "mustard" or "brown" algae, these organisms cling to the walls in sheets and add a yellow tint to almost anything that comes in contact with the pool, including swimsuits, pool accessories, and filters. They thrive when chlorine levels are low and pool filtration is insufficient. Blooms are tough to eradicate once they've started, even with proper maintenance.

Black algae:‌ Like yellow algae, black algae infestations are notoriously hard to remove, and once established, the algae colonies will continue to thrive even with correct pool care. The blooms appear as dark black or bluish-green spots between 1/4 and 1 inch wide. These algae are frequently spread through swimsuits worn in contaminated bodies of water, and they take over if the water suffers from insufficient filtration and low or inconsistent chlorine levels. To kill black algae, pool owners must penetrate its strong, protective outer layers and destroy its long roots that seep into pool plaster and tile grout.


While most algae are not dangerous to humans, some release toxins that can irritate skin or cause other health issues. Additionally, algae blooms can promote bacteria growth that can cause rashes, infections, and other medical problems. For these reasons, avoid swimming in a pool with algae until the infestation has been eradicated.


What causes red algae in a pool?

Red algae are typically introduced to a pool through wind, rain, or objects that were contaminated in another body of water. These can include items such as pool accessories or bathing suits.

How do I get rid of red algae in my pool?

Run the filter constantly, backwashing or cleaning it twice daily. Correct the pool’s chemical levels and reduce the pH to 7.0 to 7.2. Brush the sides and bottom of the pool and then vacuum it twice a day. Shock the water twice, waiting for the chlorine levels to return to normal between treatments. Once the chlorination levels have returned to normal, adjust the pH to 7.2 to 7.8. Finally, clean the pool filter and bleach any accessories, toys, or swimsuits exposed to the infected water.

Is it bad to swim in red algae?

Yes. The bacteria release toxins that may cause skin irritation, nausea, lung irritation, vomiting, and dizziness.

What chemical removes red algae?

Because red algae are bacteria and not algae, it's best to avoid relying on a pool algaecide for treatment. Instead, shock the pool using a calcium hypochlorite cleaner to kill red algae.