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Swimming fun with your own backyard pool is a great thing. More and more people are putting an inflatable swimming pool in their backyard or having an in-ground pool built right away. However, when the temperatures rise, we are often asked what to do about green pool water.

Nobody would like to swim in a green pool. When heavily polluted, the pool water quickly looks like the water in a natural pond. In fact, swimming in green pool water can even be hazardous to your health.

If you want to keep the pool water crystal clear, you have to do a few things. In this guide, we present immediate measures you can take to save pool water. We also present a number of preventive measures so that the pool does not turn green in the first place.

Why does the pool water turn green?

If the otherwise clear pool water turns green, there is exactly one possible explanation: algae.

If the pool has only a greenish haze, the algae are just making their way into the pool water. If there is a deep dark green color, the algae problem is already in an advanced stage.

Algae will multiply if not enough disinfectant is used. (Most pool owners rely on chlorine in the form of chlorine tablets or bromine as a disinfectant).

So, if algae is able to grow in the pool, it is a sign that you have been too lax lately in testing the pool water and adding chlorine.

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Pool green: clear it step by step

At the first sign of green turbidity, you must act immediately. If you wait, you’ll have a lot of work to do later and you’ll have to use more pool chemicals. Cleaning then costs more time and money. So start immediately. Then you will have something from your own pool again sooner.

Step 1: Clean the water surface with a pool net

First, remove coarse dirt and algae with a pool landing net. It is best to choose a fine-mesh landing net for this purpose, which really captures all the dirt.

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Step 2: Scrub the pool

If you have an acute algae problem, the first thing you need to do is brush off the areas that are particularly affected. If only the pool water is greenish and the pool walls or bottom do not feel slippery, it is not necessary to scrub the pool.

In the case of a heavy algae load, you should alternate back and forth between step 2 and step 3 several times to remove most of the algae. This is because when scrubbing the bottom and the walls of the pool, the visibility is too poor after a short time to continue working effectively in the case of heavily algae-infested pools.

Especially in the case of intensive algae contamination, it is important that the pool brush is very stable. For inflatable swimming pools and pools built of painted concrete or plaster, you must choose a pool brush with nylon bristles.

Owners of pools made of (unpainted) concrete better use a pool brush with steel bristles. These pool brushes are more effective at removing dirt and algae, but would damage the soft Vynil material of an inflatable pool or the paint of the concrete.

Step 3: Vacuum the pool bottom

Now it’s time to use a pool vacuum to vacuum the bottom of the pool as well as the pool walls to remove the algae. Set the filter system valve to waste and vacuum the algae out of the pool.

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A lot of water is lost in this process. For this reason, focus on the areas that are particularly overgrown with algae. If there is a lot to vacuum, you should run fresh water into the pool in parallel via a garden hose.

If only your pool water is green, but you can’t make out any real algae structures, jump straight to the next step.

Note: If you want to make your pool maintenance job a little easier, you can purchase a pool robot or an automatic pool cleaner. These self-propelled devices suck up debris, thereby keeping the pool clean. However, in case of an acute algae problem, you should not use the pool robot. The pool robot would rather serve as a chauffeur for the algae cells and distribute them throughout the pool instead of killing them.

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Step 4: Check and adjust pH

Before moving on, you need to measure the pool water pH. The ideal range for pool water pH is between 7.0 and 7.4, which is the range where the disinfectant will have its full effect.

Furthermore, with a pH in this range, no other problems will occur. Bathing in the pool is neither harmful to health nor the pool as well as the pool equipment will be damaged.

For such a measurement you can use simple pool test strips. If you want even more accurate results, it is best to get a digital pool water meter.

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If the pH is too high, you need to lower the pool pH. For this purpose, it is best to use the chemical pH Minus Granules.

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In case of too low pH, you need to increase the pool pH accordingly. The best way to do this is to use pH Plus granules.

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Step 5: Perform shock chlorination

With the pH properly set, the groundwork has been laid to perform shock chlorination. Pool shock chlorination kills all algae cells and other foreign matter.

Most pool owners rely on calcium hypochlorite pool shocker. When using it, follow the instructions on the package and calculate the amount needed. The calculated dose usually refers to shock chlorination, which is part of regular pool maintenance.

In case of heavy algae infestation, you should increase the dose. We recommend using two, three or four times the amount depending on the type of algae:

  • Green Algae: Two times the dose
  • Yellow algae: Triple dose
  • Black algae: Quadruple dose

Note: Do not use normal chlorine granules under any circumstances. For shock chlorination, use only special pool shocker, which consists of non-stabilized chlorine. The special type of chlorine decomposes under sunshine after a short time and therefore does not remain in the pool water for long. For this reason, shock chlorination of the pool water should also only be carried out after dark.

Tip: Take the opportunity to put all pool equipment and bathing toys into the pool water so that they are also disinfected.

Step 6: Backwash the filter system

The filter system plays an important role in truly filtering all algae cells from the pool water. However, before you can use the filter, you need to backwash the sand filter system. Thoroughly cleaning the filter prevents the sand filtration system itself from becoming a breeding ground for algae.

Essentially, backwashing involves turning the water flow upside down. In this way, dirt, debris of all kinds, and even algae caught in the filter are flushed out. The used pool water is discharged from the swimming pool as wastewater.

Step 7: Run the filter pump

Next, run the filter pump to filter out dirt and algae from the pool. Be sure to clean the filter regularly and replace it if necessary. Especially when cleaning a greenish pool, dirt particles will quickly accumulate in the filter.

Note: For pools without a filter pump, the procedure is difficult. Pool water with only a slight green tinge can often be made clear again with disinfectant (e.g. chlorine) and algaecide alone. In case of severe clouding or green discoloration – for pools without filtration system – complete replacement of the pool water is usually the only solution. After draining the water, you should thoroughly clean the walls and the bottom with pool brushes. Stubborn dirt can be removed with chlorine-based cleaning agents.

Step 8: Test pool water & balance pool chemistry.

After the roughest debris has been removed, you need to rebalance the pool chemistry properly. You can test the pool water with test strips, a test kit or a digital meter. Of interest are the alkalinity, pH and chlorine levels.

Ideally, the values will be in the following ranges:

  • pH: 7.2 to 7.6
  • Alkalinity: 80 to 150 ppm
  • Chlorine content: about 3 ppm

You can effectively prevent a new algae infestation only with a balanced pool chemistry. So if it is necessary, you have to readjust.

With pH Plus and pH Minus respectively, you can increase or decrease the pH value. Especially important is the chlorine concentration. The disinfecting effect of chlorine is needed to nip a new algae epidemic in the bud.

Step 9: Get in the water & prevent properly

The pool water is now clean and you can finally bathe again!

In any case, however, you should take preventive measures to avoid another algae plague.

You can find out exactly how to do this in the following.

Preventive measures: Prevent green pool water in advance

With step-by-step instructions, it should be easy for you to turn green pool water into crystal clear water.

Some tips you can implement right away:

  • Add a dose of algaecide right after cleaning. This will kill any algae still remaining in the pool.
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  • Balanced pool chemistry is the most effective preventive measure against algae infestation. Check the pool water regularly and
  • Perform shock chlorination every two weeks (weekly during the summer) to kill any foreign matter in the pool.
  • Disinfect and wash pool equipment, swim gear, air mattresses and bath toys before swimming with them in the pool to avoid introducing algae.
  • Run the filter pump for 8 to 12 hours each day.
  • Regularly check the pool walls and corners. Algae especially likes to nest in the hard-to-reach places.
  • It makes sense to add flocculant at weekly intervals. Flocculant causes the smallest particles to clump together and thus be captured by the filter. The use of flocculant is particularly useful in turbid pool water.

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  • During the summer months, it is also a good idea to regularly add algaecide after a thorough cleaning.
  • In addition to chemical measures, you should also cover the pool with a pool cover when not in use. This will prevent external debris from entering the pool in the first place.

Pool water green – clean with home remedies

Limited home remedies can be used to clean greenish pool water. We had already mentioned that pool chemistry plays a very crucial role. Most important are the chlorine concentration and the pH value.

If you do not want to rely on classic pool chemistry, you can at least adjust the pH value with the help of home remedies:

  • Normally, one lowers the pH with inorganic acids (such as pH Minus). Alternatively, vinegar can be used. One liter of vinegar per 10 cubic meters can lower the pH by 0.2. One disadvantage of using vinegar is that it is a food source for algae and bacteria. Therefore, the consumption of chlorine will increase.
  • Professional means to increase the pH are based on sodium carbonate (i.e. soda ash). If you do not want to purchase a remedy such as pH Plus, you can use washing soda. Approximately 5 grams of soda ash per cubic meter are needed to raise the pH by 0.2. However, always raise the pH value gradually.

Contrary to popular belief, vitamin C does not help against algae infestation. In the short term, vitamin C can make the water clearer, but vitamin C does not have a disinfecting effect such as chlorine.

You can effectively tackle the algae problem only with classic disinfectant (like chlorine). Just follow our step-by-step instructions. Then nothing will go wrong.

FAQ: Frequently asked questions about green pool water answered

When is a complete water replacement necessary?

If the pool is green, the cleaning measures explained above will usually help out. First, there is a rough cleaning of the water as well as filtering. After that, you need to measure the pH of the pool water and adjust it properly. Then a shock chlorination takes place and with the help of the application of algaecide the algae cells are destroyed. If all these measures do not help, you have a serious problem and a complete replacement of the pool water is inevitable.

Often, in pools without a filtration system, the pool water must be completely renewed. Light contaminants and dirt can be removed, but it becomes difficult to keep the pool water clean in the long run. Green pool water or severe cloudiness is a sign that the water needs to be drained completely.

If summer is already coming to an end, it’s not a big problem that the pool is turning green. You probably wouldn’t be swimming in the water much anyway. One consideration may be to simply leave the pool over the winter, using the pool water to water the plants in the garden in the spring. Over the winter, the chemicals in the water break down and the water quality resembles that of rainwater after some time.

It is not allowed to simply let the pool wastewater seep away. Especially if there is a high concentration of chlorine, as well as the use of algaecide, you must wait before the water can be used again to water the plants. For this reason, you should always test the pool water (chlorine concentration, pH, algaecide concentration, etc.) before discharging it.

Once the pool is empty, you can manually remove the dirt and green film from the bottom of the pool as well as from the pool walls. Use a pool brush to do this. Pool owners of liner pools, pools with painted concrete or fiberglass, should rely on pool brushes with nylon bristles.

Pool brushes with steel bristles are better at removing algae structures and debris. However, the hard stainless steel bristles can damage the soft vinyl material or paint of pools with painted concrete.

Is it possible to clean the green pool water with vitamin C?

People often turn to vitamin C as a household chlorine substitute. While vitamin C can provide clear water, it will not help against algae in the pool or heavy metals. Only through the targeted use of chlorine (or other disinfectants) in combination with a regularly running filter system can you address the problem.

For this reason, vitamin C is not a remedy that is suitable for permanent use. However, in the short term, vitamin C can be used to improve water quality.

When is the algae growth particularly strong?

Algae proliferation is favored by warm temperatures, humid air and sunlight. In addition, algae love skin flakes and body care products. Therefore, algae problems occur most frequently in the summer months.

Algae spores enter the pool through the air or dust, but also get into the swimming pool through air mattresses, bath toys or bathing suits that were previously worn in algae-contaminated waters. Algae problems can never be completely avoided in advance.

However, if you use algaecide, test the pool chemistry at least weekly and always balance it, perform shock chlorination regularly, run the filter pump daily and check the pool walls and corners for the first traces of algae, you won’t let the pool turn green in the first place.

What if the pool water is green despite shock chlorination?

Shock chlorination of pool water kills foreign organisms in the pool, but also increases the concentration of cyanuric acid. The problem with a lot of cyanuric acid in the pool is that the acid inhibits the disinfecting action of chlorine (also known as redox potential). This is because cyanuric acid ions bind to free chlorine.

The stabilized (bound) chlorine takes longer to kill bacteria, pathogens and algae. For this reason, the pool water may be green, despite shock chlorination. Unfortunately, if there is too much cyanuric acid in the pool, the only last step that will help is partial or complete water replacement.

What if the pool water is immediately green?

It can happen that the pool water is immediately green after filling. In most cases, well water was used for filling in this case.

In this case, discoloration occurs very often, which can be either green, brown or almost black due to oxidation processes with various substances, and a further side effect is the appearance of rust.

Is green pool water dangerous?

An algae infestation is a wake-up call and your action is urgent. The key is to get rid of the algae quickly. Three different types of algae like to make their way into pools. All three types of algae are – fortunately – harmless to your health.

The problem with algae in swimming pools, however, is that they are a food source for bacteria, germs as well as insects. Besides, algae also increase the risk of slipping. Additionally, an algae-infested pool is of course not a nice thing, and nobody wants to bathe in the broth anyway.

It can also be really dangerous with pets or small children. A swimming pool heavily contaminated with algae is so cloudy that in some cases the bottom of the pool is no longer visible. If a child or animal falls into the pool, it is hardly noticed. In addition, many children drown quietly.

What if the pool water is green after winter?

Of course, if the pool water is green after the winter, it’s not much fun when the pool is commissioned in the spring. No pool owner wants to have to get out the white lab coat first thing when commissioning.

The reason for the greenish color is probably that the winterizing concentrate was omitted when winterizing the pool. This winterizing concentrate is a mixture with algaecide. The concentrate prevents the formation of dirt deposits and suppresses the proliferation of algae.

An acute problem with greenish pool water after winterizing can be addressed just like other algae problems. Just follow the instructions above, and you’ll be rid of the greenish tinge in no time.

Is it okay to swim in green pool water?

Green pool water is a sign of algae in the pool. The algae itself is not harmful to your health, it is just an aesthetic problem. You are also allowed to swim in algae-infested water. But: algae can only settle in the first place if pool maintenance (and especially pool chemistry) has been neglected.

The settlement of algae is therefore also accompanied by the proliferation of bacteria as well as pathogens, which can be harmful to health. However, the algae themselves are not dangerous.

If the algae problem is not addressed, eventually the pool will “tip over.” This is because dead algae cells decompose with oxygen depletion, the oxygen content of the water drops rapidly and the pool “tips”. The pool water begins to smell swampy or putrid, and the coloration becomes darker and darker.

If the pool develops an unpleasant odor of its own, you should stop swimming in the pool. If you have problems with greenish coloration or real algae plague, better follow our instructions above. Then you will soon be able to swim in your hygienically clean and crystal clear pool again.

What if the pool water is green but clear?

If the pool water is green but clear, this is a sign of only a light algae infestation. Follow the steps above to eliminate the problem.

However, we want to emphasize that your immediate action is required. Because every day the algae continues to spread and increase the problem.

If the pool has only a greenish glow, you can usually nip the algae plague in the bud only with the help of chlorine and algaecide. A filter system is not necessarily needed.

Conclusion

In conclusion, we want to reiterate that it’s important to start fighting algae right away. If you notice that the walls of the pool become slippery and a light green coloration of the pool water appears, you must take it as a clear sign and start immediately.

Algae in the pool indicates that the concentration of chlorine and flocculant is too low. The high temperatures in the summer months favor the proliferation of algae cells, the pool “tips over” and this in turn depletes the free chlorine. All this further favors the spread of algae.

If you wait too long, you’ll soon be dealing with a real problem. More pool chemicals have to be used, cleaning becomes more expensive and in the worst case only a complete replacement of the pool water will help.

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.