Pool Care Pool Troubleshooting

How to Kill Black Algae in Your Pool?

Have you started noticing some black stains on the pool plaster or concrete lining? Wondering what it might be? Well! That’s black algae – the worst algae strain you’ll ever have to deal with in pool maintenance. It’s not only the hardest to kill but also the most harmful. So, how do you kill black algae in your pool?

The best method to kill black algae is by preventing it from growing in your swimming pool in the first place by being proactive in pool cleaning and sanitation. However, if it ever finds a way and it somehow grows inside it, here are measures you can take:

  • Scrub the Black Algae Spots
  • Brush the Pool
  • Test and Balance the Water
  • Clean the Filter
  • Shock the Pool

But remember, these are the simple ways you can kill black algae. But since it can be stubborn, you might need to repeat some steps. This post is all about this algae strain. At the end of it, you will know what black algae is, how you can kill it, if it’s harmful, and also how to prevent it from occurring in your swimming pool.

Read along!

What is Black Algae?

Black algae, which is not a true alga, looks very similar, and instead of floating on the pool water surface, they prefer rooting into porous surfaces. You will find them on plaster, concrete lining, and other pool surfaces.

Unlike true algae, which are aquatic plants, these single-celled bacteria have a dark blue-green color that becomes dark in color when mixed with water-soluble pigments.

They come as encapsulated bacteria with a hard outer shell. Because of their structure, they are tough to kill.

You cannot use the pool chemicals alone to kill them. The best solution is to use a robust chemical sanitizer to break the shell dislodging the bacteria from the surfaces and kill them.

How to Kill Black Algae in Your Pool

If you have ever dealt with plain green algae, which is a plant at one time in your pool maintenance history, you should know that was nothing compared to killing and getting rid of black algae.

Remember, it’s not a plant but a bacterial, making it more strenuous and challenging to kill. But there is good news; you can kill it with persistence in pool cleaning and sanitation. But before you can jump right in, there some materials and necessities you will need to gather.

What Do You Need?

  • Pool Brush
  • Safety Goggles
  • Pool Filter Cleaner
  • Shock Treatment
  • Chemical-Resistant Gloves
  • Digital Pool Reader, Liquid Test Kit, or Test Strips
  • Backwash Hose (if you have a sand or D.E filter)
  • Chlorine Tablet Holder
  • Telescoping Pole
  • Chlorine Tablets
  • 5-gallon bucket

Steps to Follow

Step 1: Brush the Pool

Brushing the black algae loosens it from the pool’s surfaces and puts it into the water where the shock will be able to kill it.

Now that everything you need at your disposal, it’s time to jump right in. These steps will require you to go hands-on – be ready.

The start point is to brush your swimming pool surfaces. Give it the extra brushing persistence with thoroughness and intensity.

For concrete or plaster pools, use the stainless-steel pool brush to make sure you reach into the cracks and crevices. Try to dig in to reach the black algae hiding places.

If your swimming pool is made from fiberglass or uses a vinyl liner, it’s sporadic to find black algae. If you do, use a soft nylon-bristle brush – stainless steel brush can cause damage to your swimming pool.

When you brush your pool, you loosen the black algae and allowing it to be dispersed in water for the pool shock treatment to work on it efficiently.

Step 2: Scrub the Black Algae Spots

Even though you’ve brushed your pool, you might have missed a spot or two. It’d be best to make sure all the algae is loose before proceeding to the next step. You can use a hand-held wire brush or a pumice stone to give a first-hand brushing to the surface.

You can also increase the intensity and effectiveness by using chlorine tablets. They offer excellent surface scrubbing while at the same time you expose the algae to first-hand chlorine, thus starting to kill as you clean.

Wear your protective gear, safety goggles, and chemical-resistant gloves. Take one chlorine tablet and break it in half. Use it from the broken edge to firmly scrub the black algae spots.

If you can reach the spot, use the chlorine tablet holder to hold the chlorine tablet from the broken edge. Attach the chlorine holder to the telescoping pole. Now scrub the spot with the black algae.

Step 3: Clean the Filter

Another place that the black algae might be hiding is in the pool filter. If the amount of this bacteria in your swimming pool is minimal, you can backwash your sand or D.E pool filter or rinse your filter cartridge.

However, if your black algae problem is out of hand, you’ll need to clean your filter with the ideal pool filter cleaner. If there is a lot of black in the filter, it’d be best to replace the filter media. That way, you can start afresh knowing you dealt with the problem entirely.

Step 4: Test and Balance the Water

Now that your pool surfaces are clean and the filter is algae-free, it’s best to check how the whole algae scrubbing has affected the pool chemistry.

Before you can shock your swimming pool, you’ve to make sure the pool chemistry is balanced right to ensure the pool shock works as it’s supposed to.

You have the option to use test strips, liquid test kits, and digital test kits. There is no need to get concerned with calcium hardness and Cyanuric acid now.

What matters is the pool pH, alkalinity, and your pool’s sanitizer. That allows pool shock treatment to have the best effectiveness.

Ideal Pool Chemistry Levels

Alkalinity100 ppm – 150 ppm
pH 7.4 – 7.6
Chlorine1 ppm – 3 ppm
If the chemical levels aren’t in the above levels, it’d be best to ensure they are in the suitable range.

Step 5: Shock the Pool

I did mention before those killing black algae requires more than normal sanitizer levels. That means the regular pool shock dose won’t be enough to kill it. After brushing the algae off your pool surfaces, it’s time to kill the nasty black floating in the water.

If you have ever dealt with green or yellow, you know the best treatment is triple pool shock. Now, when dealing with black algae, it’d be best to make the dose four times the usual dosage.

You will need to follow the manufacturer’s instructions to know the correct dose perfect for your pool volume. Once you have the ideal dose, multiply it by four. You can use this pool volume calculator to simplify everything for you.

The best pool shock that works with black algae is calcium hypochlorite shock. Most of the other pool shock doesn’t work with this algae, as experienced by most pool owners.

When adding the pool shock, get a 5-gallon bucket, add water ¾-full with water and dilute it. Pour everything into your pool evenly. Remember to place the pool equipment that you use to brush off the black algae in the pool water to sanitize them.

It’s always best to shock the pool when the sun sets, preferably at night. Why? Sun U.V. rays affect chlorine effectiveness, and you don’t want that when trying to kill black algae.

Step 6: Circulate the Water by Running the Pump

Having added the pool shock, you will want to disperse it to all the corners of your swimming pool to maximize its effectiveness. Run your pool pump for at least 24 hours. Your swimming pool may turn cloudy but don’t worry, that’s normal.

Step 7: Brush the Pool Again

Now everything is floating in the water is being killed by the pool shock, but there might be a few black algae left on the pool surface. So, it’d be best to brush your swimming pool twice or thrice them off the pool floor and walls.

Step 8: Clean the Filter

After the 24-hour wait is over, it’s time to clean the black algae trapped in the pool filter. The first time you cleaned the pool filter, it was to get out the live bacteria.

When you powered on the pool pump, your filter started filtering out the dead algae when shocking your swimming pool.

Cleaning it would help remove all the concentrated bits of the black algae. This time there is no need to replace the filter media or cartridge.

The best thing to do is run the filter in the backwash position with the backwash drain hose connected and directed to the perfect draining spot. If you’re using a cartridge filter, remove the cartridge and give it a nice clean using a filter cleaner.

Step 9: Shock the Pool (When Necessary)

Do you still see some black algae remains on your pool surfaces? If even after shocking your pool and all the brushing, there are some remains of these bacteria, it’d be best to shock it the second time. This time don’t use 4-times the usual dose; just double it.

Remember to run your pool, but only that this time you don’t need to run it for 24 hours; 8-12 hours will work all right. Also, brush the pool twice during this period. Brushing the pool again is crucial to prevent black algae from ever coming back.

Step 10: Test and Balance the Pool Water

Now I think all the black algae is dead. Right? If you can see any more black staff on the pool surfaces, it’s all done. The remaining part is, testing the water.

After all the work, some chemicals will surely be out of hawk. You’ll need to balance them out. Use the table I provided above to see if the levels are ideal.

In the coming days, keep a close eye on your swimming pool. Check for any signs that the black algae might be returning.

It could return even after all the brushing and shocking, especially if you missed a small spot that can start to grow again.

If black algae reappear, it’d be best to brush it off and scrub the spot using broken chlorine tables. Next, give your swimming pool a pool shock to kill the bacteria.

How Do You Know You Have Black Algae in Your Pool

We all know to this point that these black algae, the cyanobacteria, are hard to differentiate from dirt and other black staff that can grow in your swimming pool.

It starts like a tiny black dot or huge mold clumps. At times it’s more blue-green color than black. In fiberglass and vinyl liners, the black algae appear as seldom.

Here are the signs to look for when you suspect your swimming pool has black algae:

  • If you spot black or blue-green clumps or spots with raised heads and attached to the pool surfaces – nothing floating freely in water
  • It sticks to the rough areas of your pool floor and walls with a stronghold.
  • When cleaning the wall, they are sticky to the pool surface, and they don’t come off quickly with a regular brush. It might even take some effort when removing it with an algae brush.
  • When cleaning the wall, they are sticky to the pool surface, and they don’t come off quickly with a regular brush. It might even take some effort when removing it with an algae brush.
  • Unlike stains, they come off after some persistent scrapping.

It might appear even when you’re proactive in balancing your pool water chemistry, sanitize it, and ensure it stays filtered. It has other ways of getting into your swimming pool like water source.

Why Do You Have Black Algae in Your Pool?

Black algae have to be introduced in the swimming pool, just like true algae. Mostly it comes from a water source during refill or swimsuit.

If you swim in a lake, ocean, or other natural body water, there is a risk of carrying the bacteria with your swimsuit into a swimming pool. It’s pretty easy to introduce it when you swim with it.

Like all bacteria, black algae grows well in filthy water. The worst is, it doesn’t get affected by pool chlorine even if you keep the chemicals balanced. If your swimming pool filter isn’t filtering well, the risk is even more significant.

When the pool filter is clogged, there could be a buildup of dirt and black algae the can start growing there. Before you know it, your swimming pool surfaces are turning black or blue-green.

Is Black Algae Harmful?

Yes. Cyanobacteria produce cyanotoxins quite power natural poisons that can make you, pets, or other animals finding their way into the swimming pool sick.

When you swim in the water infected with the bacteria can lead to illness. However, the probability of illness severity of black algae is more when you accidentally drink the water.

If kids accidentally swallow your pool water infected with black algae, they may start experiencing anything from stomach cramps and nausea to liver damage.

And since pets can also drink the water, you might want to know they are in the same danger of getting sick if it’s infected with the black algae.

I should also mention cyanobacteria blooming in natural water bodies can also kill animals and organisms living in that water. It blocks sunlight, hoards oxygen and nutrients.

How to Prevent Black Algae from Invading Your Pool

The best and most efficient way to prevent black algae from invading your swimming pool is to prevent it from entering it in the first place.

Since it’s difficult or even close to impossible to prevent black algae from spreading, you can set parameters that prevent it from growing to save yourself from all the above troubles.

Start by ensuring black algae spores never gets to your swimming pool. Cover your swimming pool when not used to keep away the airborne bacteria spores from landing inside your swimming pool.

Never use the same swimsuits you used when swimming in a natural water body like a lake. The black algae spore available in those waters can often clench onto your swimsuit, and before you know it, you’ve introduced them in your swimming pool.

Suppose you don’t have a different swimsuit; clean and sanitize yours thoroughly before swimming with it in your swimming pool. Ask the other swimmers to do the same politely.

And remember, pets do love natural waters, and they can also bring the black algae spores to your pool. The best solution here is not to allow pets into the swimming pool. If that proves to be tough, wash them thoroughly before they can swim.

Lastly, it’d be best always to keep your pool chemistry balanced. If you do, you’ll be making the swimming pool inhabitable for the black algae. Ensure your pool filter stays operational and clean it regularly to avoid clogging, which increases the risk of cyanobacteria infestation.

Kill Black Algae – Make Your Pool Swimmable and Safe

Killing black algae isn’t easy at all. You have to scrub the surfaces with all your energy, shock it with a quadruple dose, brush again and again and clean the filter. Preventing it, however, doesn’t have to take much from you since all you need is to clean your swimsuit right, especially if you were swimming with it in a natural water body.

Restrict pets from your swimming pool, but clean them thoroughly before allowing them in the pool water if that is impossible. Cyanobacteria are harmful and can cause illness to both humans and pets, especially if they drunk infected water.

About the author

Sharif Miah

Hi! I'm Sharif, the founder of Globo Pool® and I have been working in the pool & hot tub industry for the last few years. I love to share my experiences with people & hope you are enjoying my information and lessons!

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