Swimming Pool Algae: What they are, Why they Grow, and How You Can Combat them

—Good circulation, filtration, water chemistry, and maintenance can prevent algae blooms

It’s a fact of life: The microscopic spores that grow into algae are virtually everywhere outdoors.

Spores constantly enter your backyard swimming pool via the wind, the rain, on swimsuits, or even attached to previously spore-contaminated pool-cleaning equipment. While they are typically invisible to the naked eye, they won’t be if your pool environment allows them to sprout into tiny plant colonies.

While you can’t escape these microorganisms, you can take steps to help prevent spores from gaining a foothold inside your pool and developing into algae blooms. With a three-ring protection plan—proper management of water chemistry, good filtration and circulation, and steady cleaning and maintenance routines—you can slow or halt their spread.

Algae isn’t necessarily a health hazard, but it is undesirable on several levels. Algae growth mars the beauty of your pool, and nobody wants to swim in a pool with a slimy green outbreak. Heavy overgrowth can cause cloudy water, reducing visibility—and that can pose a safety risk by altering swimmers’ depth perception.

If you don’t attack the spread of algae in your pool when it starts, a single bloom may set the stage for future outbreaks. So it makes sense to follow the adage of planning sensibly for prevention—versus tediously having to dole out a cure.

What algae are and why they grow

Algae are plant-like microorganisms that thrive in aquatic environments. They sustain themselves through photosynthesis, a process that uses sunlight to synthesize food from water and carbon dioxide. Photosynthesis involves the production of green-pigmented chlorophyll, which generates oxygen as a byproduct.

Although algae can grow in the shade, they thrive and reproduce most rapidly during continuous, warm, sunny days. And algae love water—whether it’s in a pond, your fish aquarium, or your swimming pool.

Algae needs food to survive, and every organic contaminant—from dust to leaves—that finds its way into your pool water can serve as fodder for a meal. This means that having lots of swimmers and higher levels of debris in your pool are factors that can spike algae growth.

Worldwide, there are more than 20,000 varieties of algae. However, you can narrow all of those strains down to three primary types based on color: green, black, and yellow/mustard. (Some include pink as a category, but pink “algae” is not algae at all; it’s a form of bacteria that resembles algae.)

Let’s look at some tips on how to kill algae once it’s infiltrated your swimming pool. But first, here are the best ways to prevent its entry in the first place.

How to prevent swimming pool algae outbreaks

Algae flourishes when plenty of dirt and other organic material are left unchecked in a pool. It’s tougher for algae to grow when pool water is efficiently circulating, and when water is regularly filtered and treated with the right mix of chemicals.

The pool environment is even less hospitable for algae when it’s kept clean with solid maintenance practices. What helps further is an automatic pool cleaner that continually removes debris and other organic matter from the water.

So, it comes down to this: Guarding against algae invasions fall into three main battle strategies—effective pool circulation and filtration, proper water chemistry, and consistent cleaning and maintenance. Each of these involves its own set of tactics for holding the line against algae incursions.

Pool circulation/filtration

The first line of defense against algae is how well the water in your pool water circulates and gets filtered. That’s why the pool pump is so important.

The pump serves as the heart of your pool equipment system, moving water from the pool and through the pool equipment for filtration—and for some pools, through advanced sanitizing systems like UV germicidal lights and ozone oxidation as well.  By “turning over” the pool water on a daily basis, the circulation process helps stave off algae blooms.

The pool filter itself plays a pivotal role in cleaning your pool water. Three basic types are available: sand, cartridge, and Diatomaceous Earth (DE).

In recent years, cartridges have become the most popular choice among pool buyers. Cartridges of pleated fabric help trap loose algae floating in the pool water, removing it from your pool environment. 

• Complementary cleaning and circulation equipment

To take pool cleaning and circulation to a higher level, you can ask your pool builder or remodeler about an optional in-floor system or other supplemental cleaning and circulation systems. These upgrades help make the pool far less inviting for algae to take hold.

With an in-floor system, pop-up heads in the pool floor spin through cleaning cycles, helping boost circulation throughout the pool and keeping pool floors clear of dirt and debris.

Another kind of “booster” cleaning and circulation technology comes in the form of specialty jet nozzle arrays. The installation consists of a series of strategically placed, multi-directional arrays along the pool walls.

Each array is specifically configured to shoot jets of water in certain directions. Those high-velocity jets aimed at the pool walls make the walls less hospitable for invading algae blooms.

The additional currents from both of these pool systems help boost overall circulation. And that enhanced flow of pool water helps prevent “dead zones”—sluggish, out-of-the-way patches of water (behind ladders, in corners, in the deep ends) that typically don’t receive the full distribution of chemicals and filtered water. Better circulation delivers water with a more thorough dispersion of freshly treated water.

Pool chemistry and treatments

When it comes to guarding against algae good water treatment and the right application of chemicals can make all the difference. Now, no one likes swimming in a chemical soup, but when algae strike, some short-term, high doses of water treatments will be necessary for the fight against algae.

• Balanced pH

In particular, a proper pH level in your pool is vital. The pH is a measure of how acidic or basic/alkaline pool water is.

A desirable range is 7.2 to 7.6. Algae prefer a pH level of 8.0 or higher, where chlorine is rendered less effective. Test your pool water to establish the levels, and add the appropriate amount of chlorine to increase or decrease the pH as needed.

• Water sanitizers—and alternative purification systems

Quite understandably, most pool owners have a love/hate relationship with the most widely used pool disinfectant: chlorine. The chemical is effective at eliminating algae, but it can also produce noxious odors and irritating effects on eyes, skin, and hair.

You can rely on chlorine if you like. However, a better approach to sanitation is to install ozone and ultraviolet light (UV) systems in your pool. Both neutralize a variety of contaminants, including algae.

Using ozone and UV helps slash chlorine needs up to 90 percent less chlorine while disinfecting water with maximum efficiency—albeit with fewer nasty side effects.

• Algaecides

Managing your pool water chemistry—including proper pH levels, alkalinity, and calcium hardness—is key to preventing algae infestations.

Special chemicals that specifically target algae are also available. Known as algaecides, they assist chlorine by weakening the outer membrane of algae cells. They are more effective at keeping blooms from getting started than they are at getting rid of them.

To use algaecides most successfully, first, shock your pool with a high level of chlorine while running the pump to circulate your pool water for 24 hours (keep swimmers out during this process). Then add the algaecide (follow the instruction on the bottle). It’s safe to swim in a pool containing algaecide, but do wait about 30 minutes after you have added it.

Cleaning and maintenance

The third element of the protection equation is consistent pool maintenance and cleanliness. You can achieve a spic-and-span pool with several kinds of measures.

• Interior brushing

A regular schedule of brushing your pool’s interior—even when it looks clean—helps wipe out spore colonies before they develop into blooms. Pool walls, floors, and steps are the most common places for algae to take hold, so target these spots for a thorough scrubbing.

• Automatic pool cleaners

A variety of roving suction-type and pressure-type pool sweeps are available on the market. You can also consider robotic type cleaners.

Each kind and model of cleaner offer various benefits, but all of them will help remove some of the dirt, leaves, twigs, bugs, grass clippings, plant foliage, and other organic materials that find its way into your pool. 

As explained earlier, keeping a pool clean eliminates contaminants that algae could potentially use as a food source to nourish the spread of a bloom.

• Pool vacuuming

Brushing the walls and the floor will dislodge most algae blooms. But after brushing, it’s critical to vacuum the pool, too. If you don’t, the dislodged algae can fall to the bottom of the pool and may re-affix to the interior finish.

• Vigilance

Along with maintaining good circulation, filtration, and sanitation, it always helps to keep a watchful eye on your pool. Look for the beginning of a bloom, and be sure to check less-visible areas—like pool steps or ladders—where an unwanted growth could take hold.

Keep in mind that algae blooms are prone to occur after particular events, such as runoff after a big rain storm or a pool party with 20 kids. After these kinds of activities in and around your pool, taking quick action with some pool chemicals can make all the difference in holding off an ugly black, green, or yellow outbreak.

If algae manage to breach your pool’s lines of defense, and blooms are spreading in your pool, rest easy: You can get rid of the problem, but it’s going to take some effort and elbow grease. Each strain of algae requires a different set of weapons.

Unlike what Kermit the Frog claims, it is relatively easy being green. And if you’ve got free-floating green or blue-green algae in your pool, it can make the entire pool appear green. Yick! Worse, your pool can also get green algae blooms that cling to the walls or the bottom.

Take up arms against the green monster. Shock your pool with extra chlorine, and run the pump for a full day so the circulation system can thoroughly turn over the water.

After the pool water has been turned over, add an algaecide, and run the pump for another 24 hours to allow the water extra circulation time.

Be persistent about sweeping your pool interior with a nylon bristle brush, as well as vacuuming it. And check the filter to make sure it’s clear so it can efficiently perform its job of helping clean water.

Yellow algae

Yellow/mustard algae are tougher to kill than their green counterparts. They grow on pool walls—especially ones that receive little sunshine. They may look a little like sand or pollen.

To tackle their advance, you’ll need to shock your pool with a higher concentration of chlorine. Depending on the severity of the onslaught, you may need to repeat this step two, three, or four times. Also, use a pool brush on the affected pool interiors. Be warned: You run the risk of the algae returning if you skip either step.

As with the follow-up care for a green algae bloom, it’s critical to establish a regular cleaning process to head off future visits of yellow algae.

Black algae

Here’s the good news: Black algae are the least-common type of this microorganism. The bad news: This strain is the hardest to exterminate.

An outbreak looks like little black dots or spots on your pool, most of them no bigger than a dime. What makes black algae particularly stubborn is they have tougher outer shells and strong roots that can grow into the crevices of pool walls.

Eradicating black algae will take extra measures. First, follow the same sequence of attack for battling green and yellow algae.

Then, you will need to scrub and scrub the walls with a special stainless steel brush—which can be purchased as an attachment—to break through the shells and to remove the roots. Take care to avoid scratching your interior finish.

Keeping your backyard pool free of algae

Naturally, you would rather spend time enjoying your backyard retreat than seeing it turn green or discolored in some way.

To keep your pool sparkling and swim-ready, balance your pool chemical levels, maintain water with regular cleanings and steady maintenance, and install or add modern circulation and filtration.

You can also bolster these protective measures with supplement cleaning and circulation systems, like in-floor systems or specialty wall jet nozzles.  Consider an ozone generator or ultraviolet light to replace most of the chlorine for the sanitization process.

Your reward will be days, months, and years of enjoyment, relaxation, and fresh, crystal-clear water!