Algae spores constantly enter the pool, brought in by wind, rain or even contaminated swimsuits or pool cleaning tools. When conditions are right, an algae bloom can occur in a matter of hours. These conditions include out of balance water, warm temperatures, sunlight and presence of nitrates, phosphates and/or carbon dioxide. A lack of good circulation, filtration and sanitation is usually a contributing or the primary cause of pool algae.

Algae are a living aquatic creature that multiplies rapidly on warm, sunny days. Containing chlorophyll, algae utilizes photosynthesis to grow. That is, they take in carbon dioxide and expend oxygen as a byproduct. Algae can grow in the shade or sun, but most pool algae strains need some light to grow.

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. In pools with high bather count, or pools with high levels of debris or dissolved solids, algae has a smorgasbord of nutritious food. Even the dead cellular remains of previous algae blooms provide sustenance to future generations of pool algae.

Algae are always present in swimming pools, even clean and blue pools, at a microscopic size. It waits patiently for the opportunity to bloom – when the chlorine level dips and the pH rises or the pump or filter is not operating effectively.

The first noticeable problem is that no one seems to want to go swimming. The second problem is that it requires effort and money to rid the water completely of algae. Third, once you experience a large algae bloom, it becomes easier for future algae blooms to occur. It is therefore best to use preventative pool algae chemicals and techniques to constantly control algae and prevent a bloom from occurring.

Algae can cloud and color the water, making rescue attempts difficult and reducing depth perception of a diver. Algae is not harmful to swimmers per se, but pools with algae may also be a safe harbor for pathogens like E-coli bacteria.

In addition to clogging up sanitation pathways in the water, algae also clogs up the pores in a pool filter, decreasing filter effectiveness and requiring more backwashing or filter media replacement.

It can hide deep in the crevices of a filter or in rough spots on pool plaster and tile, or behind the pool light and under the ladder treads. Some strains of pool algae will 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.

Algae create a chlorine demand in the water for itself, consuming chlorine that should be working on other contaminants. As it expels carbon dioxide, the pH level of pool water can rise.

Algae are kind of like weeds in your garden. Unsightly, unwanted space takers that create more work for the gardener, and sap up nutrients and resources from the flora we wish to grow.

There are over 21,000 known varieties of algae! In the pool business we avoid all of the complication by classifying algae by the color they exhibit.

• Green Algae:
  The most common variety, green algae will usually rear its ugly head immediately following a hazy condition in the water from insufficient filtration and/or sanitation. It is frequently found free floating in the water, although it also will cling to the walls. It reduces water clarity and is thereby distinguished from severe copper precipitation, which will impart a clear, green color to the water. Varieties of green algae also appear as "spots" on surfaces, particularly rough areas, or places where circulation is low. They also show up as "sheets", where large wall sections or even the entire pool is coated in green slime...UGH!
• Yellow Algae:
  A wall clinging variety, also called mustard algae, is usually found on the shady side of the pool. It is sheet forming, and can be difficult to eradicate completely. Once begun, a pool owner could spend the entire season fighting yellow algae. Re-infection is common, as small pockets of yellow algae may survive treatment on pool toys, floats, cleaning equipment, swimsuits or within the pool filter. This variety is resistant to normal chlorine levels and must be dealt with firmly. Hit it hard!
• Black Algae:
  Perhaps the most aggravating strain of algae, it can be extremely difficult to eradicate completely. This is not entirely accurate, but the difficulty in removing it fully is due to the strong roots and protective layers over top of the black algae plant. Black algae will appear as dark black or blue/green spots, usually the size of a pencil eraser tip, up to the size of a quarter. 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 in the presence of normal sanitizing levels and proper filtration. This form of algae commonly enters a pool inside the swimsuit of a person who's recently been to the ocean, or from contaminated pool cleaning equipment, introduced by a traveling pool guy.
• Pink Algae:
  Not really algae at all, but a form of bacteria, which appears as spots or streaks in corners and crevices. It is slow to spread and rare that it will bloom over an entire pool. Also known as pink slime or pink mold, it forms in the same manner as other biofilms, and 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. Like yellow strains, pink algae 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. Pink algae thrive when chlorine levels are low, or artificially suppressed with high levels of cyanuric acid.

Proper chemical balance and sanitizer levels will prevent many opportunities for algae to bloom. High pH and low chlorine (or other sanitizer) 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, giving algae opportunity to bloom, unless chlorine levels are increased.

General cleanliness of the pool is also important. Organic material and bacteria contribute to algae growth. Regular brushing of seemingly clean pools is not only good exercise for you, but prevents dirt from harboring in the pores of the plaster, which is a good start for an algae colony.

Using specialty chemicals or algaecides is recommended to provide a back up to normal sanitation and filtration processes and is necessary for many pools. These chemicals are described below: “Proper Filtration” is a term we throw around a lot, and it refers to quantity and quality of filtration. Most pool filters should run for a minimum of 12 hours per day, or longer if the pool filter is undersized or the filter media (sand or cartridge) is old and not as effective as it once was. 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.

• Potassium Tetraborate: This algae treatment chemical, when added to the pool water in proper dosage, 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.

• Chitin: Not an algaecide (meaning to kill algae) per se, but its properties might be called algaestatic (that is, to prevent algae growth). Chitin has the ability to coagulate and remove a wide variety of suspended materials and impurities from the water. This allows the sanitizer to more effectively kill contaminants unobstructed. It also improves the effectiveness of the filtration equipment. Sold under the trade name Sea-Klear, chitin can be a valuable weapon in your algae arsenal.

• Phosphate Remover: Phosphate removers remove phosphates and nitrates from the pool, which are a very tasty food for algae. Pools can become contaminated with phosphates from fertilizers, mulch washing in the pool, or from heavy leaf and debris loads. You can test your pool for phosphates with testing strips, and treat for removal with PhosFree, or PhosKlear.

• Filter Cleaners: Filter cleaners are useful to keep your pool filter in top condition, ejecting oils, minerals and metals that clog or gum up a pool filter, and remove 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.

• Algaecides and Algaestats:

  1. Quaternary Ammonium Compounds: A low grade type of algaecide, Quats, as they are called, will usually have "10" printed on the bottle, representing 10% active ingredient. Although available at a lower cost, quats tend to produce a small amount of surface foaming and are not known as strong algae killers. They are most effective as an algaestat, that is, as a prevention, not a cure.
     2. Polymers: Polymers are long, complicated chemical chains that behave in water both as an algaestat and an algaecide. They are available in percentage strength of 30 or 60%, are non-foaming, and work well as general, all around algae treatments. Poly-Quats are a blended compound of polymer and quats.
     3. Copper Based: Copper is a proven algaecide and algaestat. Available in varying non foaming strength of 3 - 10%. it works very well on all types of algae, but it has the drawback of staining white plaster surfaces a light blue/green color if it precipitates out of solution. Some copper algaecides are chelated, which means that agents have been added to prevent staining. Copper sulfate is often used for fountains and ornamental ponds to prevent algae, but is not recommended for use in pools.
     4. Silver Based: Silver has been shown to be an effective bacteriostat, which means that it works to prevent bacteria from reproducing. Non foaming and effective with pink algae. In high doses, reactions with sunlight can cause colloidal silver to deposit as black stains on white plaster. When using copper or silver algaecides, the use of a sequestering agent is recommended.
     5. Gold Based: Gold algaecides contain flecks of gold to fight the strongest forms of algae. Just kidding, there is no such thing as algaecide with gold in it – but at these prices, you may think so!

• Chlorine Enhancers: These are not algaecides, but work to provide a synergistic boost to pool shock when added separately, but at the same time. Sold under trade names like Green to Clean, Yellow-Out or Swamp Treat, it is effective on all types and colors of algae. Some chlorine enhancers contain sodium bromide, and some formulations are ammonia based. The addition of ammonia and lots of chlorine creates monochloramines, which act well on many types of algae. 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.

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 properly. Shut off the pool heater if you have one, to lower the water temperature. Adjust valves for optimum circulation and allow it 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.

Brush the walls and floors towards the main drain on a daily basis, and vacuum as needed. Using a flocculent may be a good choice after shocking, if the pool is extremely "swampy". 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 with fresh water, while removing as much debris as possible.

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 that will be 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.

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.

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. Clarifiers can be used to assist a struggling pool filter. Remember to run the filter 22-24 hours per day until the water clears.

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).For algae which is not suspended, but only clinging to the walls, follow the same advice above, first shock with brushing, then add an algaecide a few days later, brush again, vacuum to waste (preferred) or vacuum and then backwash the filter. Use of a steel bristled brush is recommended for algae on plaster pools (use nylon brush on vinyl). Filter, Filter, Filter!

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 spots on the walls with a trichlor tablet or stick can also be effective to 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.

If algae has been an ongoing problem in your pool for several years, you may do well to drain the pool. Many years of algae builds up dead algae cells and lots of other solids in the water that contribute to its rejuvenation. Acid wash and/or chlorine wash the pool (separately!), 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, and look for sources of contamination from fertilizers or from soils washing in the pool during heavy rainstorms. Also, look closely at the water balance and sanitation practices you have in place.

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. Next, change the filter sand if you have a sand filter or buy a new replacement filter cartridge if it is a cartridge type. Sand should be changed every 5 years (or every 2 yrs 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 DE grids, spray clean, soak in a 10:1 water/ bleach solution, rinse and replace. A well-functioning filter can prevent algae from returning.

Larger, more effective pool filters will reduce the opportunity for algae to grow, by filtering down to a much smaller particle size. This removes much of the food sources for algae, and even removes algae itself.For pool algae prevention, we need a combination of good filtration, sanitation and circulation. It may be time to consider buying a new pool filter. It is cheaper and easier to pay a little up front for more chemicals, more electricity or better equipment than all the money and aggravation spent on fighting algae blooms.