What Causes Black Algae To Grow In Your Pool?

There are many different types of algae that one can find in a pool. Black Algae is the worst. Black Algae has the most resistance to chlorine and other chemicals than any other type of algae.

The reason for this is that the black algae have tiny tentacles that grip into the porous pores of a gunite and concrete pool.

You will also get black algae in fiberglass and vinyl pools as well. The only difference is that it is easier to clean the fiberglass and vinyl pools.


Black Algae are living organisms that have hard outer shells that grow and cling to porous pool surfaces. Black algae take the form of black or blue-green spots and can be clumped together with raised heads.

They do not float in the water. Black Algae do not brush off the pool walls easily either. They are the worst form of algae and the most difficult to get rid of.


Black algae can enter your pool in various ways.

  1. From people’s bodies. If they have been swimming in a river, dam, lake or the sea, just before swimming in the pool.
  2. From people’s clothes, if they were swimming in a river, dam, lake or the sea just before swimming in the pool.
  3. Any animal that gets into pools water can have the black algae on there fur or paws.
  4. Pool Accessories, pool equipment, can all carry black algae into the water if these things are not regularly hosed off with water or washed off with diluted bleach.
  5. Swimming clothing should always be washed before it is worn to swim.
  6. Black algae will form if the pool circulation system is not functioning correctly.
  7. Black Algae will form if the pool filtration system is clocked or has not been maintained therefore cannot remove contaminants out of the water.
  8. If the PH level of the pool water is too high.
  9. If the pool pump is not running for a long enough period. Recommended period 8 to 12 Hours a Day.
  10. When the pool pump is not running every day.
  11. When the amount of chlorine in the pool water is too low.

How to remove Black Algae is a hard job, unfortunately, somebody has to do it. You can save money by doing it your self with a bit of help from the family. You can read how I got rid of black algae in my pool.

How To Get Rid Of Black Algae In Your Pool

Black algae are small living organisms that penetrate porous pool surfaces like gunite and concrete. The black algae roots grow into the cracks of the plaster where they lock-in and don’t like to let go.

There is an effective way to treat black algae and eliminate them from your swimming pool water. The problem with black algae is that it has roots that grow into the cracks of the inground pools plaster. With an above ground pool, you will get the black algae that start to form together as clumps in the water. The pool cleaning process will be very much the same for both types of pools.

This is what I have been able to do to treat black algae and stop it from returning and the pool chemicals that I have used.

Before you begin. These are the Supplies you will need.

  • Backwash hose
  • Filter cleaner
  • Filter cartridge
  • Test strips / Liquid test kit
  • Algae brush
  • Chemical-resistant gloves
  • Safety goggles
  • Chlorine tablet holder
  • Telescopic pole
  • Calcium hypochlorite shock
  • Pool Shock Treatment
  • Algaecide
  • Chlorine Tablets
  • Granular ChlorineClean your filters
  • Muriatic Acid
  • Sanitizer

1. Clean Your Pool Filters

When you have black algae in your pool, you definitely have black algae in the pool filter. If you have a DE or a Sand filter, I recommend that they are back washed and rinsed at least twice.

If you have a cartridge filter and the black algae infestation is bad then I suggest removing and rinsing the filter with filter cleaner or even replacing the filter.

2. Brush The Pool Walls And Floor

If your pool surface is gunite or concrete. Then I suggest using a very stiff-bristled brush or a stainless steeled bristled brush.

If your pool surface is fiberglass or vinal. Then I suggest using a soft-bristled pool brush.

Now that you have chosen a suitable brush for your pool surface, its time to get brushing. Now you need to brush the black algae from the sides then the bottom of the pool.

What this does is loosen the black algae from the pool wall and floor surfaces and puts it into the water.

This is where the pool shock will take over and kill the black algae.

Remember the black algae is very stubborn and grows into the cracks and tiny crevices of the pools plaster. That’s why, if after the first round of brushing you still see some black algae spots left. Brush for the second time.

If after the second round of brushing there is still black algae spots then its time to use the chlorine tablet.

Put on your chemical-resistant gloves and eye goggles. Break a 3-inch chlorine tablet in half and fit it into a Pentair Algee-Gon 3 Inch Chlorine Tablet Holder with the broken tablet edge facing outwards. Attach a pool pole to the chlorine tablet holder if necessary.

Now brush those areas that were not removed by normal brushing with the chlorine tablet. What the chlorine tablets do is apply chlorine directly onto the black algae which penetrate into the roots of black algae which start killing them off immediately.

3. Shock The Pool Water

Now that all the black algae are mixed and floating in the water it is time to shock treat the pool water. I highly recommend these pool chemicals, calcium hypochlorite shock to kill the black algae. Or you may use granular chlorine.

Follow the manufacturer’s instructions to determine your pool’s volume for the correct amount of chemicals needed for shock treatment. Then either increasing the shock level of treatment to 30 ppm or times the amount by four.

If you are not too sure how much water your pool holds?

Then you can use our pool volume calculator to work it out.

It’s a good idea to put all the pool equipment you used eg tablet holder, algae brush, pool pole leaf skimmer, etc into the shallow end so that they can be sanitized during the shocking process.

Remember its best to shock your pool at sunset or at night. If you shock your pool during the day most of the effects can be lost due to the sun eating away at the chlorine before it has a chance to kill the black algae.

I would recommend adding 1Quart of Algaecide. This chemical kills and prevents algae from growing again. This helps prevent further outbreaks of black algae.

4. Run The Pool Pump

To get the shock treatment chemicals to mix with the black algae you will need to run the pool pump for the next 24hours continuously. The pool water may turn cloudy, but that’s normal.

Continue at regular intervals to brush any remaining black algae off the walls and floor of your pool. I had to brush another 4 times to make sure there were no remaining black algae spots.

5. Re Clean The Pool Filters

When the pool pump is running all that black algae is going to be sent through the filters. You will have to clean the filters again after the 24hour pump cycle to get rid of the dead black algae.

Rinse with water or use a cartridge cleaner for the filter when dealing with black algae. It also depends on the severity of the infestation that you had.

The Sand and DE filters will need to be back washed and rinsed well to make sure all black algae spores are gone.

It is possible that the filters might need to be cleaned for the 3rd time after 48hrs have passed.

6. Check For Black Algae And Shock Pool Again If Necessary

After the initial shocking of the pool. Running the pump for 24hrs and cleaning the filters. If you still see black algae on the pool surfaces, shock treats the pool again.

When you shock the pool this time only double the recommended dose. Run the pool pump for another 24hours. Continue with the brushing. At least twice a day.

7. Test And Correct The Balance Of The Pool Water

After about 2 days your pool water should be free of all black algae. Using the correct test kit, test the water PH and Total Alkalinity chemistry levels and add the appropriate chemicals as needed to bring all the levels back to where they should be.

8. Now That The Black Algae Has Gone

I am sure you are exhausted after that job. I never want to do that again.

Regular pool maintenance every week is so important. Such as cleaning out the filters, running the pump, testing the water chemistry, vacuuming the pool, brushing the pool, etc.

This little bit of work when the pool stays clean is much better than a 2day nightmare. A clean pool is easier to maintain than a dirty pool.

Weekly maintenance keeps your pool safe for your family and pets.