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Algae in the pool, what now? – When the weather is warm and sunny, algae sprouts up quickly. In no time, microscopic algae cells become a problem that affects your entire pool.

The one spot on the pool wall or the small deposits on the steps are easy to ignore, but hesitate too long, and it will cost you a lot of money and time later.

So act before the foreign organism gets completely out of control. The sooner you take action and the more you focus on preventative measures, the lower the likelihood of an algae infestation.

Later, we’ll give you important pool algae prevention tips so you can keep algae out of your pool altogether right away.

Now, however, we’ll get right into algae removal. If algae has already taken over your swimming paradise, that’s no problem either. With our step-by-step instructions, your pool will soon be crystal clear again.

Recognizing an algae infestation

Algae usually get into pools when algae cells cling to swimwear and are carried into the pool as a result. The same applies to bathing toys such as buckets or balls with which you have been in algae-infested lakes or the sea.

The reason for the algae infestation is not important at first. The important thing now is to get rid of the algae quickly.

Are algae in the pool dangerous?

Fortunately, there are only three different types of algae that spread in pool.

All three types of algae (green, yellow and black algae) are harmless to health, but turn the pool water cloudy.

Algae also attract various bacteria and insects, as they are a source of food for them. And let’s face it, no one wants to swim in the pool water anyway.

Note: You should be especially careful with small children or pets now, because you won’t be able to see them underwater. Since most people drown quietly, it’s best to pull a pool cover over the pool.

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The most noticeable symptom of a pool with algae is discolored pool water. Based on the color, you can determine the type of algae. Either the pool water looks greenish, yellowish or blue to black.

Pool algae especially likes to grow on stairs, edges, in the corners of the pool and anywhere that is shady. Keep an eye out for these spots. This will help you determine the type of algae.

The different types of algae

  • Green Algae: Green algae is the most common type of algae. In technical language, the green algae are called chlorophyta. The name comes from the plant pigment chlorophyll. The slimy green algae settle on pool walls and the bottom of the pool. The reason for green algae growth is often poor filtration or lack of disinfectant. Typically, green algae is carried into the pool via bathing suits or toys from algae-contaminated waters.
  • Yellow algae: Pool owners often mistake yellow algae for pollen or sand. Because of your coloration, this type of algae is also known as mustard algae or brown algae. Yellow algae is chlorine resistant, which makes it difficult to remove.
  • Black algae: Black algae is the most persistent type of algae. Their roots burrow deep into plaster and concrete surfaces, making removal difficult. Technically, they are not algae at all but cyanobacteria, which multiply quickly and therefore require aggressive treatment.

Removing algae from the pool step by step

When it comes to algae in the pool, many people think directly of algaecides and disinfectants like chlorine. Yet, these chemicals are not really effective as a remedy for algae in the pool. It takes a few more steps to remove algae completely.

Our step-by-step guide is suitable for all three types of algae. There is a difference in one step. We will explain it later.

Step 1: Simply disinfect and wash everything.

Algae was somehow brought into the pool. Algae cells can get stuck on swimwear, toys, air mattresses, the pool equipment, and even the yellow squeaky duck if it has been in algae-infested water.

It would be very annoying if you go to great lengths to combat the algae infestation, only to have it carry back into the pool. So gather all of your swimsuits, swim toys, swim trunks, pieces of your scuba gear and plastic balls and clean them.

Wash the clothes in the washing machine. For the rest of the items, use a strong detergent and scrub them down with a brush.

Tip: A cool trick is to simply place bulky or large pool accessories, in the pool. The following steps will disinfect the pool water. You clean your toys and equipment at the same time, clever or?

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Step 2: Test the pool water

Check the pool water on a weekly basis. Before you take care of the algae problem, you should also perform a water test. Use simple test strips or a test kit for this. The most important values are chlorine content (FC), pH, and alkalinity (TA).

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Step 3: Correct the pool chemistry

When pool chemistry is balanced, the values are in these ranges:

  • pH: 7.2 to 7.6
  • Alkalinity: 80 to 150 ppm
  • Chlorine level: approximately 3 ppm

If necessary, you should readjust at this time. Balanced pool chemistry is an important requirement for the following steps. For example, a too high pH or too low alkalinity would inhibit the action of the chlorine. If all values are correct, you can jump directly to the next step.

You increase the pool pH with the agent pH-Plus and the chemical pH-Minus lowers the pH. Alternatively, home remedies can be used.

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With alkalinity, it’s a little more complicated. There are chemicals to raise the alkalinity, but lowering it is more difficult. Excessive alkalinity can only be lowered by (partially) replacing the water or by intermittently adding pH minus to the deepest part of the pool.

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If the level of chlorine is too low, you need to add chlorine granules (e.g. in the form of chlorine tablets). In case of algae infestation, you do not need to artificially lower the chlorine concentration if it is reasonably in the right range. The chlorine level will decrease over time anyway.

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Step 4: Brush like there’s no tomorrow

Scrub the algae from the pool walls. Do this thoroughly, this will allow the disinfectant to penetrate deeper into the remaining algae. Focus your work on the areas where there is a particularly large amount of algae. By scrubbing intensively, you can quickly remove pool algae from the bottom.

In shady places, corners and hard-to-reach places, the algae especially likes to grow. Even a short time after you start brushing, this will limit your visibility in the water. If this is the case, you should alternate between steps 4 and 5 until you have removed most of the visible algae.

Your pool brush should be as sturdy as possible. Ideally, this will have steel bristles. On the other hand, if you have a vinyl pool, you will need softer bristles such as nylon.

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However, with a steel pool brush, you can be especially effective in removing pool algae from the bottom.

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Step 5: Manual Vacuuming

Set the valve to waste (“Waste”) and then vacuum the algae out of the pool. You will lose a lot of pool water during this process. Therefore, focus on the areas that are particularly overgrown with algae.

If there is really a lot to suck up, you should run fresh water into the pool in parallel via a garden hose.

Note: In regular pool maintenance, automatic pool vacuums and pool robots are handy gadgets. However, if there is an algae bloom in the pool, you should do without these helpers. The reason for this is that the robots rather serve as a chauffeur for the algae and distribute them throughout the pool, rather than killing them.

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Step 6: Do a strong shock chlorination

For a proper shock chlorination, you need calcium hypochlorite pool shocker – and not too little of it. Follow the directions on the package to determine the dose for a normal pool shock. Multiply that dose by two, three or four, depending on the type of algae you’re dealing with.

  • Green algae: Two times the dose
  • Yellow or dark green algae: Triple dose
  • Black algae: Quadruple dose

For shock chlorination, do not use your normal chlorine granules, but pool shockers with non-stabilized chlorine. This chlorine will decompose in the sunshine after a short time. This is the reason why you can perform this step only after dark.

Tip: Remember to put your pool equipment in the water now to disinfect it.

Step 7: Filter the pool algae out of the water.

Check the water level, then set the valve to “filter” and turn them on. Now run the pump for at least eight hours.

Step 8: Brush and shock again.

Before you shock chlorinate again, you should again scrub down the pool walls. Focus on the areas where the algae first appeared.

Black algae in particular takes deep roots. Even if the algae appears to be removed, it may reappear later.

A single dose is sufficient for this final shock chlorination.

Note: You may find that the water is still cloudy the next morning. This cloudiness is likely a side effect of the last shock chlorination. Use clarifiers to clear them up quickly or just wait for the water to clear up on its own.

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Step 9: Clean the filter

Clean the filter. Under no circumstances should you rinse it out. After all, the last thing you want right now is to gradually flush the algae back into the pool water. Therefore, clean the filter thoroughly with filter cleaner.

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Note: Before you get back in the pool now, you should test the pool water one last time. Due to the numerous measures taken, the pool chemistry may have become unbalanced. Pay particular attention to the chlorine level when testing.

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Detect algae in the pool early

You can use flocculant to remove algae if you detect algae in the pool early. Flocculant saves you a lot of work and money by allowing the chemical to bond with the algae particles and make it easier to suck them out of the pool.

If you only have a mild problem with green algae, treatment with flocculant is sufficient. In case of serious algae infestation and already greasy pool walls, you’d better follow the regular cleaning schedule.

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Step-by-step instructions

  1. Turn off the pump.
  2. Set the valve to recycle. This will quickly mix the flocculant with the pool water.
  3. Add the recommended dose of liquid or powdered flocculant.
  4. Turn the pump back on and let it run for at least two hours.
  5. Turn the filter pump off again and let the water sit overnight. During this time, the flocculant binds to the algae cells, precipitates and accumulates at the bottom of the pool.
  6. The next morning, set the valve to waste (“Waste”) so that dirty water does not flow back into the pool.
  7. Connect the waste hose.
  8. Vacuum the pool. Proceed slowly and carefully so you don’t stir up debris from the bottom too much. If the water gets too cloudy during this step, you will need to pause briefly and let the particles settle again first. Always add water with a garden hose when vacuuming.
  9. If there is a heavy algae load: brush the walls of the pool.
  10. Do a shock chlorination with a double dose of pool shocker.
  11. Run your filter for at least eight hours.

Prevent algae in the pool and other tips

By now, at the latest, you may be wondering why algaecides haven’t even been mentioned in this guide. The name is somewhat deceiving, because in fact algaecides are a much more effective measure in prevention than in directly killing the algae.

In conclusion, we want to give you some tips on how to prevent an algae infestation. Some tips of which you can implement immediately after removing the pool algae.

  • Add a dose of algaecide after cleaning to kill any algae left in the pool.
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  • Maintain a balanced pool chemistry.
  • Run your filter pump 8 to 12 hours a day.
  • Perform shock chlorination at least every 2 weeks to control foreign organisms in the pool.
  • Wash and disinfect pool equipment, swim gear, swim toys, air mattresses, scuba equipment parts and co before you go swimming with them in the pool to avoid introducing algae.
  • Check and repair the walls and corners of your pool. Cracks offer algae the ideal hiding place, which at the same time is almost impossible to clean.

Special case saltwater pool algae

Many people think that algae in a saltwater pool is different from algae in a chlorine pool, or even pools that are disinfected with bromine or active oxygen. However, this is a misconception.

So, salt water pool algae are also green algae, mustard algae or black algae. Accordingly, if you want to remove the algae from the salt water pool, you should proceed in the same way as described step by step in this article. 

If you get into the situation that the saltwater pool is heavily algae and cannot be saved, then you should drain the water and fill the saltwater pool with fresh water.

A final word.

Algae is sneaky. Even regularly maintained pools with balanced pool chemistry can become infested with algae. High temperatures, humid air, and lots of precipitation all encourage algae infestations – just to name three factors you can’t control.

The most important thing is to act immediately when you notice algae in the pool. Algae grows day by day, making it increasingly difficult to remove. Be thorough when cleaning. If only a small number of algae spores remain, they can quickly flourish again.

Removal isn’t always easy, so make sure the algae can’t spread in the pool in the first place. Make algaecide part of your regular pool maintenance to effectively keep algae out in the first place.

Larry has been a true water rat since childhood. Pure pleasure turned into a passion. That's why he is the first point of contact for friends and acquaintances when it comes to pool-related problems. He is an integral part of the PoolHandbook editorial team.