What Causes Red Algae in Swimming Pools?

There are 21,000 known species of algae, but very few of them are red, and none of those are found in swimming pools. When swimming pool owners see red or rust-colored "algae" in their pool, what they are actually seeing is a type of bacteria. The same is true of the dreaded pink slime—it is not an algae at all, but a type of bacteria.

Swimming pool with clear water.
Clear blue swimming pool all ready for summertime.

Both forms of bacteria are introduced to pools by wind, rain, swimmers, or pool accessories. Shocking the pool with high doses of disinfectant will kill any bacteria that remains after you vacuum and backwash the pool. Once the problem is resolved, you can prevent a recurrence through diligent pool maintenance.

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. These bacteria build up in the pool water and on surfaces like ladders, pool covers, and inflatable pool toys. If your pool chemicals are out of balance, these bacteria grow and multiply rather than dying off. 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 from your garden hose.

Eliminating Bacteria

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

  1. Run your filter 24 hours a day, backwashing it twice a day until the bacteria is gone.
  2. Balance your pool's pH and chemical levels.
  3. Brush any areas where the bacteria are visible and then vacuum the pool.
  4. Shock the pool with chlorine or another accepted sanitizer.
  5. Bleach your bathing suit and wipe down pool toys with bleach.

You may need to repeat this process a few times to get rid of all the red bacteria in your pool. Swimmers can return to the pool when the bacteria are gone and the pool's chlorine level returns to between 2 and 4 ppm.

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 minute 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 your swimmers take a quick shower before climbing into your pool and keep a close eye on your pool water's chemical balance.

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