Skip to content


Home / Blog / What Causes Swimming Pool Algae?
Causes Swimming Pool Algae

What Causes Swimming Pool Algae?

Algae spores enter your pool all the time. Wind, rain, or even contaminated swimsuits and pool cleaning tools can carry spores. When conditions are right, an algae bloom can occur in a matter of hours.

These conditions include out-of-balance water, warm temperatures, and sunlight. The presence of nitrates, phosphates, and carbon dioxide can spur their growth. A lack of adequate circulation, filtration, and sanitation is usually a contributing or even primary cause of pool algae.

What is Algae?

Algae are living aquatic creature that multiplies rapidly on warm, sunny days. Like plants, algae contain chlorophyll and utilize photosynthesis to grow. That is, they take in carbon dioxide and give off oxygen as a byproduct. Algae can grow in the shade or sun, but most pool algae strains need light to bloom.

Algae need food to survive, and in a swimming pool, there is no shortage of tasty food for algae. Nearly every contaminant or windblown speck of dust can feed pool algae. It is impossible to keep dust and debris out of your pool at all times, so there are likely nutrients in the pool that causes swimming pool algae to feed. 

High bather counts, levels of debris, and dissolved solids provide algae with a smorgasbord of nutritious food. Even the remains of previous algae blooms can feed future generations of pool algae. The minerals and vitamins from past algae colonies are providing food for future populations of algae to prosper.

Algae are always present in swimming pools, even clean blue pools, but usually only in tiny amounts. Algae wait patiently for the opportunity to bloom. And when the chlorine level dips and the pH rises, or the pump or filter is not operating effectively, it causes swimming pool algae to explode and take over!

What Causes Swimming Pool Algae [infographic]

What Problems Can Algae Cause?

The primary problem with algae is that no one wants to swim with them. Swimmers want crystal clear, blue water to relax and play in the pool. 

The second problem is that it requires effort and money to remove them. Unfortunately, once you experience a large algae bloom, it becomes easier for future algae blooms to occur. Therefore, it is best to prevent pool algae chemicals and techniques that control it before it can bloom. It is much easier to prevent the causes of swimming pool algae from flourishing in the first place than it is to eliminate the blooms after they occur. 

Algae can also be dangerous. They can cloud and color pool water, making rescue attempts difficult and reducing visibility for divers. Algae create a chlorine demand in the water, using up chlorine that should be working on other contaminants. Once algae forms, the pool turns into an easy safe harbor for pathogens like E-coli bacteria. Algae are kind of like weeds in your garden. Unsightly, unwanted space-takers create more work for the gardener and sap up nutrients and resources from the plants we wish to grow.

Kids play in the pool, doing handstands and diving for sinking toys for fun with friends! Adults make lounge in a floating seat to cool off in the hot summer sun while reading a book. No one wants to be in a pool filled with bacteria and gross green algae everywhere!

Filter Damage

Algae also clog up the pores in a pool filter, decreasing filter effectiveness and requiring more backwashing or filter media replacement.

It can hide deep in a filter’s crevices or in rough spots on pool plaster and tile, or behind the pool light and under the ladder treads. Some strains of pool algae send roots into the plaster and slowly degrade and stain pool surfaces. Algae can even grow under vinyl pool liners, on the walls, or floor beneath the liner.

What types of Algae are there?

There are over 21,000 known varieties of algae! Many things cause swimming pool algae, but it is also important to know the types of algae that occur. In the pool business, we avoid all of the complications by classifying algae by the color they exhibit. 

Green Algae

The most common variety 

Causes- insufficient filtration or sanitation

Appearance- free-floating spots in the water, clings to pool walls in “sheets,” green slime

Problems- reduces water clarity

Yellow Algae

Also called mustard algae

Causes- low chlorine levels, poor filtration, especially in the pool’s shady side

Appearance- wall-clinging, sheet-forming and can be challenging to eradicate. Yellow tint can be on anything, pool floats, filters, swimsuits.

Problems- Once begun, a pool owner could spend the entire season fighting yellow algae. Reinfection is common. This variety is resistant to normal chlorine levels and must be dealt with firmly. Hit it hard.

Black Algae

The most aggravating strain of algae, it can be extremely difficult to eradicate. 

Causes- improper filtration, low and inconsistent sanitizing levels. This form of algae commonly enters a pool inside the swimsuit of a person who’s recently been to the ocean or from a contaminated pool.

Appearance- dark black or blue/green spots, usually the size of a pencil eraser tip, up to the size of a quarter. 

Problems- It has strong roots and protective layers. Their roots extend into the pool plaster or tile grout, and unless the roots are destroyed, a new head will grow back in the same place. The heads also contain protective layers to keep cell-destroying algae treatment chemicals from entering the organism. Like yellow algae, black algae can bloom even under normal sanitizing levels and proper filtration.

Pink Algae

Not really algae at all, but a form of bacteria. Also known as pink slime or pink mold.

Causes- prefers to attach itself to smooth surfaces, out of the way from your pool cleaner and pool brush, in areas of low flow or circulation. Low chlorine levels or artificially suppressed with high levels of cyanuric acid.

Appearance- spots or streaks in corners and crevices. Not all over the pool, just small spots.

Problems- requires a high level of chlorine to treat effectively, and for best results, throw in all of your pool toys and floats, suits, and cleaning tools into the pool during algae treatment. Removing ladders and pool lights for a good scrubbing is also good practice to prevent reinfection.

How Is Algae Prevented?

Proper chemical balance and sanitizer levels will prevent many opportunities for algae to bloom. High pH and low chlorine can give algae a great start. Using cyanuric acid (stabilizer or conditioner) to protect your chlorine from the sun has the added effect of suppressing chlorine activity, allowing algae to bloom unless chlorine levels increase.

Lack of cleaning causes swimming pool algae. The general cleanliness of the pool is essential as organic material and bacteria contribute to algae growth. Regular brushing of seemingly clean pools is a great habit to practice. It prevents dirt from sticking in the plaster’s pores, which is where many algae colonies start.

Automatic Pool Cleaner

Poor filtration causes swimming pool algae. “Proper Filtration” is a term we throw around a lot, and it refers to the quantity and quality of filtration. Most pool filters should run for a minimum of 12 hours per day. It may need to run longer if the pool filter is undersized relative to the size of the pool, or if the filter media (sand or cartridge) is old and not as effective as it should run.

Poor circulation can also play a role, especially for larger pools with inadequate plumbing or pump size. Using an automatic pool cleaner can help circulation immensely. Using specialty chemicals or algaecides is recommended to provide a backup to regular sanitation and filtration processes and is necessary for many pools. 

These chemicals are described below:

Potassium Tetraborate

When added to the pool water in the proper dosage, this algae treatment chemical prevents algae from converting carbon dioxide into the fuel it needs for growth.

For swimming pools, you can use a product manufactured under the trade name Proteam Supreme.


Not an algaecide (meaning to kill algae) per se, but its properties might be called algaestatic (that is, to prevent algae growth).

Chitin can coagulate and remove a wide variety of suspended materials and impurities from the water. This allows the sanitizer to kill contaminants more effectively. It also improves the effectiveness of the filtration equipment.

Phosphate Remover

Phosphates do cause swimming pool algae. Phosphate removers, like its name, remove phosphates as well as nitrates from the pool, which are delicious food for algae. Pools can become contaminated with phosphates from fertilizers, mulch washing in the pool, or heavy leaf and debris loads.

Filter Cleaners

Filter cleaners are useful to keep your pool filter in top condition, ejecting oils, minerals, and metals that clog up a pool filter and removing dead algae to prevent reinfection.

Although not useful in killing algae, a pool filter cleaner used regularly will help your filter trap the particles that become algae food, in addition to trapping algae itself. It must be cleaned out to flush out the algae and its food fully. A filter that is not cleaned does cause swimming pool algae to bloom. 

Chlorine Enhancers

These are not algaecides but work to provide a synergistic boost to pool shock. Sold under trade names like Green to Clean, Yellow-Out, or Swamp Treat, it is effective on all algae types and colors.

Some chlorine enhancers contain sodium bromide, and some formulations are ammonia-based. Adding ammonia and lots of chlorine creates monochloramines, which act well on many algae types.

Sodium Bromide creates bromamines in the pool, at least temporarily. Chlorine enhancers have the distinction of working well in pools with high levels of cyanuric acid, over 50 ppm – which, as mentioned earlier, makes free chlorine less potent and reactive.

If all of this sounds like too much information, time, and effort, trust the professionals at The Pool Butler!

Too Late to Prevent It…how Do I Kill Algae?

First off, balance your water, paying particular attention to pH, as your chlorine is much more active in the low end of the range, 7.1-7.3. Secondly, check that your filter and pump are operating correctly.

Shut off the pool heater if you have one to lower the water temperature. Adjust valves for optimum circulation and allow the pump to run 24 hours a day until the pool clears.

Turn on pool cleaners to help stir things up. Backwash as necessary, but only when pressure rises by at least 5 psi or the flow rate is noticeably diminished.

Shock Treatments

For suspended green algae, shock the pool hard. Put in as much hypochlorite as it takes to turn the pool a cloudy, bluish/gray color, which generally requires about 30 ppm of free chlorine.

The higher your cyanuric acid level is, the more pool shock is required to overcome the sluggish effect of stabilization. A general amount would be between 2 to 5 lbs of granular pool shock per 10,000 gallons of pool water – using more when cyanuric acid levels are above 30 ppm or when the algae bloom is especially aggressive. A poorly completed shock treatment causes swimming pool algae to continue to grow. 

Test the water the following day for pH and chlorine. If the chlorine level is still very high, that’s good – if it has dropped to zero within 24 hours, you may have missed the mark and will need to shock the pool again, using slightly more this time.

Brush the walls and floors towards the main drain daily, and vacuum as needed. Using a flocculent may be a good choice after shocking if the pool is still swampy.

Draining a Pool

If you cannot see the bottom of the pool and it is filled with leaves and debris, it may be wise to drain the pool, acid wash, and refill it (plaster pools only). It is nearly impossible to restore clear water to a pool that is very dirty with debris.

Another option is to drain half the water and refill it with fresh water while removing as much debris as possible.

After the chlorine level has come down below 5 ppm, add an algaecide and brush the pool again. When it all settles, vacuum the pool (to waste, if possible).

Test and re-balance the pool water after it clears. You can use clarifiers to assist a struggling pool filter. Remember to run the filter 22-24 hours per day until the water clears. Not running the filter long enough is a common cause of swimming pool algae.

Steel Bristle Brush

For algae that are not suspended but only clinging to the walls, follow the same advice above. First, shock with brushing, add an algaecide a few days later, brush again, vacuum to waste (preferred), or vacuum followed by backwashing the filter.

We recommend a steel-bristled brush for algae on plaster pools. Use a nylon brush on vinyl. And filter, filter, filter!

For black algae, the brushing part is extra important. You must tear through the protective layers so the chemicals can destroy the plant from the inside out. Pumice stones work well to knock off the heads of black algae. (Don’t forget to vacuum them up later and backwash them out of the filter ASAP).

Also effective on the black algae nodules is sprinkling crushed pool tablets over the spots (of course, if they’re on the wall, this is next to impossible). Rubbing the marks on the walls with a trichlor tablet or stick can also help knock off the heads and get some chlorine directly on the plant.

Follow up with a dose of copper algaecide or high-strength polymer algaecide. Be sure to use only pool chemicals; never use pond chemicals or agricultural herbicides in a swimming pool.

When Brushing Isn’t Enough

If algae have been an ongoing problem in your pool for several years, you may do well to drain the pool. Many years of algae can build up dead algae cells and lots of other solids in the water that contribute to its rejuvenation. Acid wash or chlorine wash the pool to kill the roots of the algae embedded in rough plaster.

For pools with repeat algae blooms, you may also test for phosphates in the pool. Look for causes of swimming pool algae, such as sources of contamination from fertilizers or soil washing into the pool during heavy rainstorms. Also, look closely at the water balance and sanitation practices you have in place.

Filter Cartridge

Next, if you have a sand filter, change the sand out. If you have a cartridge filter, buy a new replacement filter cartridge.

Change the filter sand every five years (or every two years if you use Baquacil), and cartridge filters should have new elements every few years, depending on size. 

If you have a D.E. filter (good for you!), you should remove the D.E. grids, spray clean, soak in a 10:1 water/ bleach solution, rinse and replace. A well-functioning filter can prevent algae from returning.

Another item to look at is the size or condition of the pool filter. Far too many pools out there have marginal filter systems meant to run 24 hrs per day. When these systems get old and tired, or the new owners only run it 12 hrs per day (or less), algae can take hold and take over.

We know this is a lot of information! It takes a lot of knowledge and understanding to correctly treat swimming pool algae. In the end, it is always a good idea to trust the professionals at The Pool Butler!

The Pool Butler