The Ultimate Guide to Pool Algae

What is Pool Algae?

Pool algae is an unofficial classification for a wide variety of microorganisms that grow in swimming pools. Despite the naming convention, different types of pool algae are not that related in a biological sense. Pretty much if it isn’t an animal, an insect, or a person in the pool, it gets called algae.

What Causes Pool Algae?

Although a very diverse group, most algae growth (often referred to as “blooms”) are a result of insufficient sanitizer (i.e. chlorine for most pools). Things like increased temperature, more sunlight, high bather load, and water balancing can contribute to additional chlorine loss. This brings the sanitizer level below the tolerance for most algae to grow. This is why algae typically become an issue when the weather gets warm and everyone starts swimming.

Pool Algae Types

There are thousands of species of what we colloquially refer to as “algae”. There are 4 types of pool algae by their biological classification: stramenopiles (“mustard algae” and “white water mold”), viridiplantae (“green algae”), cyanobacteria (“black algae”) and eubacteria (“pink algae/slime”).

Stramenopiles: Mustard Pool Algae and White Water Mold

Although very different in appearance, these are the two most closely related types of algae. Stramenopiles are classified as having unequal flagella (think “tails”) at some point in their lifespan. They wiggle these tails as a means of propulsion (eww gross I know).

Mustard Algae

Mustard or yellow algae is the most common form of algae in pools. They are most common in the “sunbelt” of the U.S., stretching from California, along the southern border, all the way to the mecca of mustard algae: Florida. Typically, mustard algae grows on the walls of the pool in shadier areas. They tend to be easy to brush off but reattach quickly to the walls of the pool.

White Water Mold

White water mold is a less common form of algae that can be found across the U.S. White water mold is a cousin of Mustard Algae. However, instead of preferring to grow on walls, it tends to be present in the water itself, appearing as a white tissue paper like debris. Although less common, it tends to grow more quickly than Mustard Algae.

Viridiplantae: Green Pool Algae

Algae are a type Viridiplantae (lit. “Green Plants”), and the second most common form of algae. Unlike Stramenopiles, they typically only have one flagellate or tail for propulsion (yes, still gross).

Green algae prefer to grow in more humid environments – most commonly in the South East to North East when the weather gets warm. Typically Green algae blooms will begin to grow after longer neglect than the above Stramenopiles. They typically will grow on the walls and be present in the water.

Although they may have a lower tolerance for chlorine and sanitizer, blooms of green algae usually have more biomass which can require more sanitizer to kill once present.


Cyanobacteria: Black Pool Algae

Black Algae is probably the third most common algae but comes in first in being the most difficult to kill. Black algae is actually a type of bacteria, called Cyanobacteria. Cyanobacteria are some of the oldest known living organisms on earth.

Appearing as small bluish-green spots on the surface of the pool, the cells grow in long chains. To spread, these long chains wiggle in the water. Usually, black algae appear in corners or areas that are “dead spots” where there isn’t very much circulation. They are very difficult to brush off, usually requiring a 50% Nylon, 50% stainless steel brush.

Cyanobacteria have very thick gelatinous walls – much thicker than other types of algae. This makes them extremely resistant to oxidation by sanitizer. On top of this, when under threat Cyanobacteria releases “Akinetes”; extremely durable yet dormant cells that can survive the harshest conditions (even being able to survive acid washing). When conditions allow these cells will reactivate and start growing again. This is why a pool that has black algae will always seem to get again. While you can never truly get rid of the dormant cells, with prevention you can keep them from appearing again.

It’s also worth mentioning that Cyanobacteria in its active form is toxic. Usually, this isn’t much of a concern since it’s so diluted by the volume of water in a pool. Just don’t lick it – it can make you sick.


Eubacteria: Pink Pool Algae, Pink Pool Slime

Last on our list is Pink Algae, sometimes referred to as Pink Slime. This is a different form of bacteria – specifically of the species of Serratia. This is also a common bacteria in hospitals – responsible for infections around I.V. and similar sites on patients.

While less common and much easier to kill compared to Cyanobacteria, this species of bacteria grow very quickly once present. It tends to grow in shaded areas of the pool: around fixtures, behind pool lights, and along the bottom of walls.

What can make it difficult to kill is the sheer amount of it in a pool; once visible odds are there is a ton more hidden in areas that are very difficult to get too. The volume of biomass can also suck up a lot of chemical energy when trying to kill it all.

Pink algae pose even greater health risks than even cyanobacteria – Pink Slime can infect people. When dealing with pink slime in a pool, make sure to cover any open wounds and wash your hands thoroughly afterward. And avoid swimming.

How do I get rid of pool algae?

Now you have an idea of the different types of algae, you probably saying to yourself “great – but how do I get rid of it?”

In general, you want to get as much of what you’re dealing with into the water. This might be just a simple light brushing if it’s mustard algae, some elbow grease with a 50/50 brush if it’s black algae, to having to fully remove your pool light and scoop it into the water if it’s pink slime. Once in the water where it’s exposed, it’s usually a process of adding an algaecide, then a heavy dose of shocking, and then letting the filter take the dead stuff out of the water.

Just remember, if you’re dealing with a seriously bad pool you might be fighting more than one type of algae.

For a more thorough how-to, go check out our article on our 2 Day Algae Clean Up where we go into more detail.