How to Get Rid of Black Algae in Your Pool (7 Easy Steps)

Black algae in the pool are the horror of every pool owner and are considered harmful to health. What are the needed steps for combating black algae, and how do you permanently get them out of the pool?

The black algae in the pool must first be brushed off with a steel brush. After brushing the floor and the walls, it is necessary to shock the pool water with chlorine. An algaecide and flocculant can aid the filtration of the black algae in the last step.

In this article, I explain how to remove black algae from the pool and which preventive measures help keep this type of algae away permanently.

What are black algae?

To prevent black algae in the future, you should know what black algae are.

The well-known algae in your swimming pool are plant-like organisms that live in the water and carry out photosynthesis.

These use the light energy of the sun to produce various organic substances. In other words: no algae can survive without light.

The black algae (also known as blue algae) are neither plants nor algae. They are wrongly called algae, but they belong to the group of cyanobacteria. Source.

But how does the black algae develop in the pool?

How do black algae appear in the pool?

Black algae prefer neutral and slightly alkaline waters.

Despite optimal pool maintenance, black spots – which look like mold – can form in the pool.

It is likely the dreaded black alga.

You may see black algae as black spots in the pool.

These are introduced into the pool water either by humans or pollen count.

First, this stubborn type of algae grows in the joints of the pool and then spreads further and further in the pool.

Therefore, a pool in a poor structural condition is an ideal nesting place for the black algae to multiply.

PVC pools or stainless steel pools have no joints and were welded – The black algae is rarely found in these pool forms.

But don’t worry.

You are not completely powerless if you have a problem with this type of algae in your pool.

I explain below in this article, preventive measures against black algae in the pool.

How to kill black algae in the pool?

Due to their nature, black algae are considered particularly resistant and cannot be eliminated with simple home remedies.

That is why you will find a list of all the necessary products to combat black algae.

  • A pool brush with steel bristles (not for GRP and liner pools)
  • Algaecides that are used specifically against algae
  • Chlorine granulate or liquid chlorine for shock chlorination
  • Chlorine tablets to permanently produce free chlorine
  • Pool floater for the chlorine tablets
  • Pool test strips for the water quality
  • Safety glasses and plastic gloves

This pool equipment is sufficient to clean a pool infested with black algae.

Some advisors even recommend replacing the sand in the sand filter system beforehand.

If your pool is heavily infested with black algae, it can’t do any harm to replace the filter sand in the sand filter system.

#1 Vacuum the pool floor

First, you should completely vacuum the coarse dirt from the pool floor with a pool vacuum.

The dirt and the fine suspended matter impair scrubbing with the steel brush and would only be whirled up.

In the worst case, when you brush, you will miss some places where the black algae are sticking.

To prevent this from happening and keep the view clear, I may recommend a battery-powered pool vacuum.

You can skip this step if there is no heavy soiling from leaves or sand in the pool.

#2 Clean the pool with a steel brush

Chemical agents are not enough because black algae are difficult to remove without a suitable brush due to the slime layer and its complex shell.

Scrub the pool thoroughly, and a second time with the steel brush if necessary.

Small black spots on the floor or the walls can be removed with a targeted application of chlorine granulate.

This is how it works: Break a chlorine tablet and rub the black algae with the edge.

Wear gloves or use a holder for the tablet.

As soon as the algae turn brown, they have died and can be brushed off.

Tip: Do not use a steel brush in a liner or pvc pool. In this case, you use a normal pool brush and scrub longer.

#3 Use an algaecide against the black algae

The algaecide is used to combat specifically algae in the pool and is available in different qualities:

  • Slightly foaming algicide
  • Strong foaming algicide
  • Non-foaming algicide

To inhibit a new growth of black algae in the pool, I would recommend adding some of the algaecides to the pool water.

You can improve the algaecides effect by increasing the chlorine value – I recommend adding approx 125 g of chlorine per 25 m³ of pool water.

However, the pool algaecide is far from sufficient. It is only used – as already mentioned – to reduce the growth of black algae.

The pool shock in the next step is crucial in the fight against black algae.

#4 Sprinkle the pool floor with chlorine granulate

Suppose the bottom of the pool is mostly infested. In that case, you should sprinkle it completely with chlorine granulate – switch off the pool pump beforehand.

Do not aim at individual spots and scatter the soil over a large area.

I recommend using chlorine granulates.

Suppose the black algae are predominantly on the walls. In that case, a targeted application is not possible.

You have to scrub the algae with a steel brush before using the chlorine granulate – you can rub the algae with a chlorine tablet beforehand.

Keep brushing until all the black algae fall to the floor.

Then it would be best if you sprinkled the pool floor with chlorine granulate as described.

My tip: Wait for a windless moment when sprinkling and wear protective goggles and a protective mask.

After sprinkling with chlorine granulate, let the pump rest for 24 hours to build up a chlorine cushion.

After the 24 hours have elapsed, the black algae are sucked off with the filter system or a pool vacuum.

But that’s not all.

A pool shock is needed to inhibit further growth and kill the last black algae, which I will explain in the next chapter.

#5 Perform a pool shock against black algae

To kill any black algae, the pool has to be shocked, i.e., over-chlorinated.

Non-stabilized liquid chlorine may be suitable for shock chlorination in the pool. The amount used depends on the concentration.

  • With a chlorine content of 80 g/l , 10 liters of liquid chlorine are used per 25 m³ of water.
  • With a chlorine content of 150 g/l, 5 liters of liquid chlorine per 25 m³ of water are sufficient.

I recommend this liquid chlorine that you can buy online.

Furthermore, the liquid chlorine in combination with a dosing system is wonderfully suitable for permanent disinfection of the pool.

You can avoid splashes during the use of the pool shock by pouring the liquid chlorine into the flow of the return nozzles.

After the pool shock, the pump should run for 24 to 36 hours until the water is clear again.

Measure the chlorine content every day and keep it above 2 ppm.

The filtration can be increased by using a flocculant in your swimming pool.

During this time, scrub the basin with the brush so that all algae are removed.

You can only take a bath again when the chlorine values ​​are below 1.5 ppm.

My tip: The liquid chlorine is irritating, and you should also wear appropriate protective equipment here. If you don’t currently have protective equipment on hand, at least wear sunglasses.

Are black algae dangerous?

The black algae (blue algae) is considered dangerous and has no place in a pool.

Since these types of algae produce cyanotoxins (one of the strongest poisons), the following symptoms of the disease can occur:

  • nausea
  • Stomach cramps
  • Liver damage

Symptoms usually occur when adults or children swallow pool water contaminated with black algae.

But pets can also get sick if they drink water that contains black algae.

Avoid the pool and forbid its use as soon as you have discovered black algae!

Swimming is only possible again when the black algae have been completely removed from the pool.

I explain how you can take preventive measures against black algae in the pool in the following chapter.

Preventive measures against black algae in the pool

A short filtration time is one of the most common reasons for an algae problem.

Furthermore, algae are always formed when disinfectants are not used properly.

That means: You can prevent algae simply by taking care of the pool.

The following values ​​should be observed when treating the water:

  • pH: Between 7.2 and 7.6
  • Alkalinity: The total alkalinity should be between 80 and 120 ppm
  • Calcium hardness: A value of 200 to 400 ppm is optimal

An electronic water test helps you precisely determine all pool water parameters.

Suppose you notice that the pool water is slowly becoming cloudy or algae are forming. In that case, this indicates that the filtration is too short.

The solution: Scrub the swimming pool with a pool brush and let the pool pump run longer until the water clears again.

You don’t have to use any extra chlorine here either.

In addition, you should brush the pool regularly so that the algae do not continue to grow.

This process also favors the effectiveness of disinfectants in combating black algae.

Copper sulfate may be suitable as an additional algae killer. However, this agent is based on heavy metals and is therefore considered to be questionable.

A far better alternative for the preventive measure against algae is silver.

Products containing silver for pool maintenance are safe for bathers and can be added directly to the water.

In addition, silver-containing care products are highly effective in combating algae.

What to do if the disinfectants don’t work?

It is difficult to determine that disinfectants are not working properly.

The consequences are often not announced in the pool, and the pool water suddenly tips over.

Only the shock chlorination described above can help.

But professional pool maintenance is not the only way to prevent black algae.

I described above that the black algae nestle in the joints of the pool.

If you do not successfully kill the black algae in the pool with my described method, then only one thing will help.

The water has to be drained completely, and the basin has to be renovated.

Therefore, you should not only clean the pool after the winter but also examine the pool for damage and cracks. If necessary, repair the cracks in the spring before filling your pool.

If you have any further questions, don’t hesitate to contact me at @contactswimfool on Twitter, Facebook, or Pinterest. I am happy to help you.

With that in mind, have fun in the pool.

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Hi. I'm Max Berg. I've been in the pool industry since 2015 and have always felt drawn to water. I'm the author behind swimfool.com, where I share my years of experience in pool maintenance and give helpful tips on keeping a swimming pool or hot tub clean. My tips reduce the costs of water treatment and protect the environment.

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