Algae Guide for Chlorine Pools

Chlorine System Troubleshooting Guide


Potential Causes

  • Poor Filtration: A damaged or dirty filter provides an excellent breeding ground for algae. Also if the pool water is not being properly circulated, ‘dead spots’ can occur in the water in which algae can start growing.
  • Water Balance: When the free available chlorine level is not maintained between 1 – 3ppm, or algaecide and shock (oxidizer) is not used properly, there is a higher incidence of algae. Chlorine controls algae growth, algaecide kills the algae, and shock oxidizes organic materials, which can be a potential food source for the algae.
  • Poor Housekeeping: By not keeping your pool clean and free of leaves, dirt, and other debris, you can provide areas where algae can grow and thrive protected from chemicals. Weather can play a significant role in the contamination of pool water because wind and rain can carry large amounts of leaves, dirt, fertilizer, and algae spores into the pool, so proper care should be taken to minimize the pools exposure to such elements.


  1. Vacuum all visible algae and debris to waste. The more you can remove initially, the faster your pool will become clear again. Vacuuming to waste is very important, as it removes the contaminants from your pool and eliminates any chance of them passing through your filter and returning to the pool.
  2. Brush down the sides and bottom of your pool.
  3. Adjust the pH of the pool water to between 7.2 – 7.8, if necessary. If the pH is significantly out of this range, the sanitizer will not work effectively.
  4. The next step can be done in either sequence, depending on the time of day.If it’s daytime, add the proper amount of the appropriate algaecide for your algae type. Circulate the pool water. Follow with a chlorine shock treatment that evening and continue to run the filter overnight. If you start in the evening, add a chlorine shock treatment that evening, and let the filter circulate overnight, and follow with the appropriate algaecide for your algae type the following day. The pool water should be looking much better at this point.

Two things to remember

  1. The chlorine shock treatment will be used up rapidly and it would not be unusual for the pool to have no chlorine in it by the next day. It is very important to continue with your daily method of chlorination.
  2. Very often with algae, after the pool is shocked and algaecide has been added, the water will become blue, instead of green, but the pool will be cloudy. What has happened is that when algae dies, it breaks up into millions of tiny pieces. These are too light to sink to the bottom, and too small to be filtered out. If this is the case see the Haze Guide.

Some last thoughts

  • It doesn’t happen very often, but it is possible that a particular type of algaecide may not be effective against your algae. If the algaecide does not succeed, switch to another type of algaecide, not just another brand of the same type. There are specific types of algaecide geared towards specific forms of algae. Ask the experts at Pettis for help determining which kind is best for your specific problem.
  • If you have a problem with recurring algae blooms, it could signify that you may have a high level of phosphates in your water. Phosphates are a food source for algae, and come from a variety of sources including source water, fertilizer, some laundry detergents, and tile cleaners. Pettis Pools can test your phosphate level, and you can remove it by adding either Ultima Nix or Revive! phosphate removers.


Since algae are always present, and grow when conditions allow, the best course of action is preventative maintenance.

  • Filtration: Check your equipment regularly to ensure it is in proper working order. Chemically clean your filter at least twice a season. Be sure to run your filter at least 8 – 12 hours daily (24 hours per day is ideal), and that your return is pointed downwards in a circular pattern. If you run your filter on low speed Check the skimmer basket regularly to remove any debris that can restrict water flow. Backwash weekly or when pressure rises 8 – 10 psi over normal operating levels.
  • Water Balance: Be sure to add algaecide and shock weekly. Test your chlorine, pH, and alkalinity levels often as chlorine becomes less effective when pH is out of balance. Bring a sample to Pettis Pools & Patio to have a free water test done regularly.
  • Environment & Bather Load: Be sure to brush pool sides and bottom weekly. Vacuum accumulated leaves, dirt, and other debris as necessary. Using the pool cover can also be helpful in keeping out leaves and debris, but be sure not to leave the cover on for extended periods of time as this can lead to algae build up.