Algae in Pool but Chlorine is High

Good advice for pool care is preventative maintenance and it doesn’t have to be complicated. If you know how and why algae grow in your pool then you can head it off at the pass so to speak before it gets out of hand but sometimes you got to think outside the box. How can Algae grow in a Pool even though the Chlorine is High?

  • Poor H2O circulation; low flow or dead spots in the pool
  • Poor H2O balance; pH, Alkalinity, Calcium
  • High Cyanuric levels
  • Poor H2O sanitation; low or inconsistent free chlorine (FC) levels
  • Poor H2O filtration; short filter run times or an ineffective filter
  • Warm H2O
  • Sunspots
  • High levels of phosphates


Algae are always present in swimming pools, even clean and blue pools, and contain algae at the microscopic size. It hides patiently for the opportunity to bloom when the chlorine level dips and the pH rises or the pump or filter is not operating effectively among other scenarios BOOM!


Algae in Pool but Chlorine is High


Your swimming pool turned green but your chlorine residual is high. Everything you understand about chlorine and algae is thrown out the window. The mystery answer is that the stabilizer (CYA)  available in the chlorine product can buffer the effect of Chlorine. Is your pool Balanced?

One of the most overlooked reasons for this to happen. The CYA will protect the chlorine from being used all at one time which will defeat the purpose of super-chlorinating. 


High Cyanuric Acid and Phosphate Levels

Cyanuric acid, or CYA as it is commonly called (and also called pool stabilizer, pool conditioner, or chlorine stabilizer), acts as that buffer, protecting your chlorine from those hungry UV rays. You can get it in liquid or granule form, or mixed with chlorine tablets or sticks ( trichlor) and in chlorine shock (dichloro).

As a result, you must ensure you properly check and adjust the levels of sanitizer in your pool. The ideal range is between 80 and 120 parts per million. If too high cyanuric acid level and it’s above that, you need to partially drain the pool and then add more water this will dilute it.

This is true unless they are maintained together in the proper ratio. Super-chlorinating the green pool water to bring up the free chlorine levels to high levels over 10ppm can correct the situation and balance and kill the green algae in the swimming pool water.

The reason the pool turned green in the first place was that the Free Chlorine levels slipped down below the proper levels for preventing the algae from starting.

What’s the answer-More Chlorine-Keep the chlorine levels high and consistent! High doses of chlorine usually quickly kill the algae when the levels are properly balanced indicates a low-level al algae. When the chemicals are unbalanced 


High pH High Phosphate High Chlorine Levels


There are a few reasons why your chlorine is not killing the algae in your pool.

  • High pH: Chlorine does not work well in water with a high pH. The recommended pH range for chlorine to sanitize effectively is between 7.2 and 7.8. In waters that have a higher pH than this, chlorine can lose up to 90% of its effectiveness. As a result of this, algae can grow unaffected.  Though you might be shocking your pool and sanitizing it routinely, your pool will still remain cloudy with algae. In general, high pH encourages the growth of algae.
  • High phosphate levels: Phosphates are basically food for algae. This means that though you might be sanitizing your pool if the phosphate levels are high, the algae will keep being fueled. They continue to bloom because they are getting nutrients. It is essential to reduce your phosphate levels so the algae are starved, and then your chlorine works more effectively.


  • Or bring your FC-free chlorine levels up to par and there won’t be any microbes (Algae) to feed.
  • Chlorine lock: A chlorine lock is a situation whereby, despite adding more chlorine, the chlorine in your pool is still rendered useless. This situation can come about when there is excess cyanuric acid in your pool.
  • Cyanuric acid is what forms pool stabilizers that you use to extend the lifespan of chlorine in your pool from tablets or granular chlorine. However, this stabilizer can have drawbacks when used in excess.



Does Adding Chlorine Kill Algae in Pool Water


Here is the thing. Test for Free Chlorine (FC) Having high total chlorine with low free chlorine makes your pool a perfect home for algae and other microbes in pool water. What kills algae in your pool is the available free chlorine.

If you notice that the chlorine in your pool is no longer effective, it could be a result of low free chlorine in the pool, which will require a shock. It could also be a result of the chlorine lock caused by the stabilizer in the pool or an unbalance swimming pool.

If you are experiencing green water or an algae outbreak, you have to increase your free chlorine concentration to a breakpoint of over 30 parts per million. The severity of the algae growth will also determine how much shock you will use.

For light green pools, you can administer 1 pound of pool shock for every 10,000 gallons of pool water and run the pool filter. For a very dark green pool, you might have to use up to 3 pounds of shock per 10,000 gallons of pool water. 



    • Common Causes for Green Pool Water

    Cyanuric Acid Levels

    If you’re using chlorine pucks in the pool water or stabilized pool chlorine, there’s a chance that you have excess cyanuric acid levels in your pool. Keeping your cyanuric acid levels at the recommended levels is a balancing act (pun intended) on its own.

    Too low a cyanuric acid level will also lead to your chlorine being burned away by the UV rays of the sun, but too high of a cyanuric acid level will make your chlorine ineffective, causing green pool H2O even if you have chlorine residual in your pool.

    pH Levels

    Another reason for getting green pool water, even if you have great pool chlorine residual levels, is having a high pH level. High pH levels, much like high cyanuric acid levels, can make your pool chlorine ineffective and algae are growing in the pool water or spa.

    You can get away with a pH level of 8.0, but the higher it goes from there, the more ineffective your pool chlorine will be. This is fairly important to note especially if you’re using a saltwater chlorinator since saltwater chlorination typically drives pool water to higher pH levels than traditional chlorination with liquid or granulated chlorine.

    Phosphate Level

    Phosphates are not something that is commonly tested (or included) in many pool water test kits. Even if you have the right free chlorine levels, having a high phosphate level can still lead to green pool water. 

    Phosphates in pools are what we consider an algae superfood, so even if you have the proper pool chlorine levels, you can still end up with green pool water. Phosphates naturally come from organic pool debris, rain, and runoff from your lawn and sometimes pool cleaners.



How do you lower Combined Chlorine

When you measure the water in your swimming pool, the amount of combined chlorine in the water should be less than 0.5 PPM. If the numbers are higher, the pool may need to be shocked to get rid of the odor..…………………………… more

Algae on Pool Walls


Another form of Algae growing in swimming pool water is when the chlorine is reading high but algae grow on the walls or the floor in streaks. If you’re not sure if there are algae in your pool, you can do a simple visual inspection to determine if it is present.

The most common pool algae will be a greenish hue and will be slimy to the touch. Pool water algae can also be blackmustard, or additional colors.

Normally these types of algae on pool walls are easy to remove because they are loose. In chlorine or bromine pools, algae grow when free chlorine is low, and or pH has drifted. In a salt-water pool, algae can get a foothold if the salt level is low or if the system has a hiccup. Free available pool Chlorine indicates a low level of algae.

Large algae attached to walls or the floor prefer hot weather and stagnant water, so if your pool goes unused for a period of time during the summer, you may come back to find an algae invasion in streaks. Algae growing on the walls or bottom of your pool is due to:

  • Poor pool water circulation
  • Not enough sanitizer in pools
  • Poor pool water filtration 
  • Poor water balance-Cyanuric-pH-Calcium levels
  • Use a good reliable test kit and check the levels 
  • Shock the pool with good chemicals
  • Brush the walls and floor of the pool-sweep gently towards the pool’s center-
  • Vacuum-then filter pool water


*Keep in mind that the hot sun spots in the pool will burn off the effective chlorine in the water, so while you may think you’re putting in the right amount of pool chemicals, adding chlorine to stay in balance but your pool’s composition may have slipped below the critical point, especially in areas of a pool.

I had a huge pool exposed to the sun and ran the filter at night to save money at a lower rate. Run the filter for an hour or so when the sun is at its highest in the afternoon, especially in the heat of summer.

*Occasionally, I dealt with streaks of algae in the deep ends of my pool. Not because of an in-balance in pool water chemistry but because of circulation problems and heat.




What is Combined Chlorine

Combined Chlorine is the portion of chlorine in the water that has reacted and combined with ammonia, such as nitrogen-containing contaminants, and other organics that come from a swimmer’s perspiration, urine,…………………………………read more


JimGalloway Author/Editor


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