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 creatures that multiply 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.

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 creates 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 a 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.

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