Why Is My Pool Green & How Do I Clean It?

Green Pool Quick Answers

Nothing will spoil your summer fun, like waking up to find that your pool has turned a sickly green hue. This is the result of algae growth, which can happen in swimming pools when the pH balance is thrown off. If the problem gets bad enough, it can render your pool completely unusable.

an inground swimming pool with green water in it
The water in this pool is green as a result of algae growth and pH imbalance.

But don’t worry! There are quick and easy steps to solve this problem during pool maintenance to clean a green pool and restore its chemical levels, so the water is crystal clear again in no time. Read on to find out exactly why your pool turned green and how to clean a green pool.

Why is my pool green?

As we mentioned above, the green color in your pool is most likely caused by algae. This likely means that your pool water has a chemical imbalance. In a healthy pool, algae never gets a foothold because it is killed off by chlorine – one of the essential chemicals in your pool water. But if the chlorine levels in your pool run low, you will probably have green pool water at some point.

Beyond a chemical imbalance, several other factors that can cause (or exacerbate) algae growth in your pool. These include:

  • Low pH levels
  • Poor water circulation
  • Warm weather
  • Filter issues

Make sure to regularly check and service your pool equipment to prevent these issues from becoming critical.

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Types of algae that can grow

You need to know the exact type of algae growing in your pool because certain algae species are more challenging to treat than others.

Green algae

Green algae is relatively common and the quickest type of algae to form in pool water. Fortunately, it’s also the easiest type of algae to treat. Green algae has a slimy coating. It shows its first signs of growth in small patches – typically near your pool’s spool’s steps or in its corners. Luckily, the slimy green coat can be easily dislodged by brushing it, making the algae easier to kill with a “shock” treatment. (More on that later.)

Yellow algae

Also known as “mustard algae,” yellow algae is notably more difficult to treat than green algae. While mustard algae also appears as a slimy coat, its lighter color makes it more difficult to spot. It’s also not broken up as quickly as green algae. That means even if you do get mustard algae brushed off of your pool walls, it’s likely to attach to another location in your pool. Not to worry, though. As with green algae, as long as you disturb the slimy, protective layer by brushing it before you shock the pool, the chemical treatment will do the rest of the work.

Black algae

The most difficult algae to treat in your pool water is black algae. Not only does black algae make its own food, allowing it to grow on its own, but it also embeds itself deeply into concrete surfaces. This makes it extremely difficult to break through the algae’s protective layer and even more difficult to dislodge it from the concrete it’s attached to. Using a stainless steel brush works best for black algae, as it is tough enough to dig through those layers and expose the algae to the chemicals in the pool shock.

What happens if I don’t fix this soon?

Not only does a green swimming pool look unattractive and uninviting – it can lead to other issues, which may be very costly. When algae is present in a pool, it can clog your filter in no time. When a filter gets clogged, the circulation of chemicals (like chlorine) that fights bacteria slows down or stops. This can lead to additional hazardous bacterial and fungal growth within your pool water. 

In other words, this is a problem to deal with ASAP. The longer you wait to take action, the more likely you are to have a clogged filter, circulation difficulties, and spiraling sanitation issues on top of your already algae-filled pool water. Here are six simple steps to help you get rid of green pool water:

A six-step process for cleaning a green pool

1. Remove any debris

First, use a skimmer net to remove any algae debris you find on your pool’s surface. Once the visible debris is removed, you can search with the skimmer net for debris floating deeper into the water. Although this process might be a little difficult if the water is murky, it is important because algae can feed off debris floating around in the water.

an inground swimming pool with green water and brown leaves in it
When removing algae debris from your pool you should also remove any floating leaves or other debris.

Note: If you are unable to see 6-8 inches below the surface, chemically treating your green pool water may not be an option. In this case, you may need to drain the pool entirely.

2. Brush your pool’s walls

Before you shock your pool, thoroughly brush the walls of your pool to loosen up the algae. Be sure to brush the ladder, the sides of the steps into the pool, and any other nooks where algae may be growing. The most common types of pool algae – green and yellow algae – have a very slimy coat that acts as a protective layer. When you brush the algae off of the walls, you are also breaking up these protective layers. Doing so will allow the harsh chemicals in your pool shock treatment to kill the algae more efficiently.

3. Check and adjust pH and chemical levels

Once you have removed the debris and brushed the walls, the next step is to measure the pH and chemical content of your pool. To do this, you will need a complete water chemistry testing toolkit. Pool Calculator – a handy app for iOS and Android – makes it simple, allowing you to monitor and adjust your pool chemistry with ease.

If you have algae growing in your pool, you most likely have low chlorine levels and a high pH in your water. The key to killing off your algae is to shock your pool, which we’ll describe in the next step. First, however, you need to lower your pool’s pH to a normal level. (The pH of your pool simply describes how acidic or alkaline it is.)

Ideally, you should aim for a pH level of 7.2 or below. To lower the pH levels, you will want to use a pH decreaser such as muriatic acid – about a gallon should do the trick. Once you have added the muriatic acid, wait a few hours and test the water again to ensure it has reached a healthy pH level.

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4. Shock your pool

The next step to getting rid of green pool water is “shocking” your pool with a large amount of chlorine. You do need to be mindful of how much shock you use though. If you use too much shock you might end up with green hair from chlorine oxidizing hard metals in the water, like copper. If you don’t use enough, you risk your pool water staying cloudy and unsafe. Typically, shocking a pool requires anywhere between 30ppm-50ppm of chlorine (parts per million) to kill off any living algae in the pool. You can do this with calcium hypochlorite, also known as granular chlorine, which is a powdered form of the chemical. Liquid chlorine will also work as a pool shock agent.

Regardless of the form of chlorine you use, make sure to disperse it as evenly as possible when adding it to your pool water. Once you’ve added the chlorine, ensure that the filter pump is turned on and running so that the water can circulate throughout the entire pool. This will ensure that the chlorine is distributed to all areas of the pool, killing all the algae. 

Depending on the type and amount of algae growth in your pool, you may need to repeat this shock process several times to successfully clear the water. If necessary, you can also add algaecide during the pool shock process to help kill the algae as quickly and efficiently as possible.

5. Pump and filter your pool water

Once the shock has done all of the dirty work, you should see your pool clear up somewhat. Still, expect the water to be cloudy after the shock treatment is done. To clear away this cloudiness, you’ll need to pump the water and continuously run your filter for eight hours at a time at minimum. Also ensure that you backwash your filter a few times to wash out any remaining debris that would slow down this filtration process.

a white cartridge pool filter next to an inground pool
Here is an example of a pool cartridge filter you might be using for your pool.

6. Maintain chemical balance

Finally, to keep the algae from coming back, it’s essential that you maintain the chemical balance in your pool. That means tracking and managing your pool chemistry and pH levels on an ongoing basis to keep your swimming conditions as sanitary and safe as possible. 

Given that algae can appear and grow within less than 24 hours, it is important that you stay vigilant! Ashift or drop in just one chemical can disrupt your pool’s overall chemistry and allow algae to thrive. Having done all this work to clean the green from your pool, the last thing you end up at square one again and have to repeat the whole algae-cleaning process.

Goodbye green pool, hello clean pool

They say an ounce of prevention is worth a pound of cure – and it’s true, whether you’re talking about your own physical health or the health and cleanliness of your swimming pool. 

Now that you know where pool algae comes from and how to clear green pool water, daily pool maintenance is the name of the game. Staying aware of your pool chemistry with simple apps like Pool Calculator will save you time, energy, and money in the long run – and will keep your pool clean (and blue) for years to come.

Sources:

https://www.homedepot.com/c/ah/how-to-get-rid-of-algae-in-a-pool/9ba683603be9fa5395fab90f66e6267

https://www.thespruce.com/pool-cleaning-algae-colors-types-2736565

https://www.medallionenergy.com/blog/fix-green-pool-5-easy-steps/

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