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Why is My Pool Green and How to Clean It?

Imagine having a beautiful pool with crystal-clear water all-year round, only to find out that the water turns green. Why is my pool green? How can I return the water condition to normal? You probably ask these questions when your pool no longer has the inviting blue color.

Algae is the most possible cause for green, cloudy pool water. Green algae are the most common one, which floats in the water or clings to the pool walls.

Black algae come as dark, slimy spots, which are harder to clean. Mustard algae look like floating sand, usually gather at the bottom of the pool.

Green algae are the easiest to remove with common chlorine treatment. Black and mustard algae are harder the remove because they are chlorine resistant. You must scrub them vigorously to “break” their outer layers. This way, the chlorine treatment will be more successful.

Protecting the pool water from algae depends on proper pool maintenance. Some pool owners are surprised to find green water even after chlorine treatment. Shock treatment (adding a large amount of chlorine) also doesn’t work if the water condition is less than ideal.

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Algae are not the only organisms that ruin your pool water. Mold can grow in the pool with a poor filtration system. Other contaminants, such as debris, dust, ammonia, and other contaminants also contribute to green, murky, smelly pool water. Flourishing bacteria can also cause water discoloration.

Surprised of finding green water in your pool? Follow this guide to understand the cause and solution.

Causes of Algae and Mold in Pool Water

Swimming pool turned green because of algae and mold
Swimming pool turned green because of algae and mold

Algae and mold can grow even in a chlorinated pool if the condition supports them. A brand new pool water is also green when algae and mold is already in there from the first place. Here are several possible causes of algae growth in pool water.

1. Wrong pH Balance

A normal swimming pool should have pH balance scale between 7.2 and 7.8. Lower pH scale will turn the water acidic, damaging various components. Higher pH scale will encourage bacteria to flourish, resulting in discoloration.

2. Warm Weather

Algae love warm weather, and they will flourish during warm weather. Exceptionally warm season can also encourage the growth of tougher algae, which are harder to kill.

3. High Phosphate Level

Phosphate can enter your pool through organic materials or debris. If they are left unattended, or if your filter doesn’t work properly, the materials will contribute to the phosphate level in the water.

The result is an ideal breeding ground for algae. If you don’t stabilize the phosphate level, the water will still look green even with a shock treatment.

4. Lack of Maintenance

Swimming pools with poor maintenance often have discolored water. The owners may rarely treat the water with chlorine. Poor physical maintenance, such as scrubbing and vacuuming the surfaces, can also create a good place for algae, bacteria, and mold to grow.

5. Clogged Filter System

Your pool filter system will not be intact forever. Poor maintenance and old age can lead to clogging, which prevents is from straining debris, algae, spores, and other contaminants. It results in stagnant water, which allows algae, mold, fungi, and bacteria to thrive.

When your pool water turns green, it is important to find out the possible reasons behind it. If your pool is old enough or used frequently, you probably need multiple solutions to return the clarity of the water.

Testing and Shocking the Water

Checking the quality of water can be your first step of fixing a green pool. Use several testing kits such as strip and filtration to get the detailed reading of chlorine level.

The free chlorine level (the one that is still uncontaminated) must be above 1 ppm. When it is below 1, you must conduct a shock treatment.

Make sure the water has a low pH level before you conduct the shock treatment. Consider the type of your filter before doing a shock treatment.

Use liquid treatment products for a pool with filtration or sand filter. Granular chlorine treatment products are better for a pool with DE (Diatomaceous Earth) filter. 

If you suspect the presence of phosphates, remove them first using phosphate removing products. They usually come in a liquid form.

Removing phosphates gives the pool water a “silky” feel when your skin touches it. Phosphates are also responsible for eye irritation and preventing the effectiveness of shock treatment.

Checking the Filter

Routinely check your pool water filter
Routinely check your pool water filter

When the filter is the main cause of green water, there are several ways you can fix it. 

1. Check the Filter Components

The first thing you must know is the presence of unseen damages.

Are there any cracks or faults that prevent the filter from doing its job? You should call a professional to do the job. If the filter is too damaged, you can just replace it from the beginning.

2. Clean and Unclog the Filter

The simplest way to fix the filter is by cleaning and unclogging it. Perform a backwash to remove debris buildup from behind the filter membrane. Remove any large debris and pieces, including twigs and leaves.

3. Perform a Daily Cartridge Filter Cleaning

If you use a cartridge-type filter, you need to clean it every day until the water is clean. Wash it from top to bottom using a hose, but don’t use the high-pressured one (it damages the soft filter elements).

Perform this cleaning every day until the water comes out clean. If it no longer produces clean water after several days, replace it.

4. Clean the DE Filter Professionally

DE filter is hard to clean because you must dismantle it. Unless you are a professional, you should give this task to a pool company. The company that installed your pool should be able to do the job.

After doing all things necessary to fix the filter, try running it for 24 hours. See if there are any changes in the green water. You probably need to run the filter for 24 hours every day until the pool finishes its cycles and the clean water is back.

5. Using the Algaecide

Algaecide is the most extreme solution for removing algae. You should only use it if the free chlorine level in the pool falls below 5 ppm. 

Tips to Choose an Algaecide

There are many algaecide to choose from
There are many algaecide to choose from

When you buy an algaecide, choose the one that kills the specific algae type. Green water is mostly caused by green algae, so you can buy any products that kill green algae.

Better yet, try products that kill green, red, and black algae, since they target all types of common algae with the price of one product. If your pool water is yellow or red, buy a product that has an antibacterial feature.

Check the product features in the label. If you need a quick result, choose products that say instant results. They usually remove algae within 24 to 48 hours at most (other products take weeks to remove algae from standard pools). 

How long an algaecide protects your water? Check the label to find out.

Some products can protect your water for three months, while others only last for three to four days. Also, since not all algaecide are suitable for swimming pools, find products that have “Pool and spa friendly” or anything with the word “pool” on the label.

1. Algaecide Recommendations

There are several popular algaecide products that get good reviews in 2019, which are:

HTH Pool Algaecide Super Algae Guard 60

This concentrated algaecide has a 60 percent strength, perfect for daily use. The product removes and prevents algae, keeping your pool clear and clean.

HTH Pool Algaecide reacts well with the pool water salt system. The formula is nonfoaming and you can swim immediately after applying it.

Kem-Tek KTK Pool and Spa 60% Concentrated Algaecide

Kem-Tek KTK is a great algaecide for all types of pool and filter. The nonfoaming solution is ideal for daily use. In the ideal pool condition, you can swim immediately after applying this product to the water.

Last update on 2019-11-08 / Affiliate links

McGrayel Algatec Super Algaecide

McGrayel Algatec is a fast-working algaecide solution. It immediately kills algae and prevents them from multiplying. You can clean green algae in 24 hours, but black algae need around 10 days to clean with this product.

Last update on 2019-11-08 / Affiliate links

PoolRX Algaecide Unit

PoolRX Algaecide Unit is perfect for practical regular cleaning. You can place the product in a pump basket or skimmer.

The product kills algae without adding dangerous chemicals into the water. Each application can last for about six months before the effects start to disappear.

Last update on 2019-11-08 / Affiliate links

Clorox Pool and Spa Algaecide Xtra Blue

Clorox Algaecide is one of the cheapest products for cleaning the water. It works wonder on green, yellow, and black algae. It also stops algae from growing, keeping the water crystal-clear. Clorox Pool and Spa is suitable for a maximum 25k-gallon pool.

Last update on 2019-11-08 / Affiliate links

In the Swim Super Pool Algaecide

In the Swim is a perfect algaecide to kill various types of algae, including the yellow and black ones. The product contains 23.5 percent of copper, making it a fast-acting and effective product. The nonfoaming formula allows you to swim after applying the product into the water.

Last update on 2019-11-08 / Affiliate links

SeaKlear 90-Day Algae Prevention and Remover

SeaKlear is suitable for prevention and removal, which lasts for 90 days. The formula is safe for a regular pool and spa pool. SeaKlear is effective for green, blue, yellow, red, and black algae.

Make sure to follow the instructions when adding algaecide into the water. Don’t use it if the water pH level is normal.

Last update on 2019-11-08 / Affiliate links

Cleaning the Algae Manually

Manual algae cleaning is important if their presence is very visible, especially if they form buildup or firm layers.

If your pool has a severe algae problem, you can start cleaning them manually before using other methods. Use a wire brush to scrub any spots with algae buildups. If the pool surface is made of vinyl, scrub it with a nylon brush.

After applying algaecide, you probably will see floating algae. Help your filter by removing them from the surface, using a net or pool vacuum.

Scrub the pool surfaces again to break the rest of them. Add a flocculent product if there are too many algae particles. This product binds the particles and turns them into larger masses, easier for cleaning and vacuuming.

How to Prevent Green Water

Now that your pool water turns crystal-clear again, make sure the green color never comes back. Follow these steps to keep the pool water clean.

Improve Pool Circulation

Poor circulation system will cause stagnant spots, which algae love. Create a small whirlpool by attaching a directional nozzle to the pool’s return pipe and positioning it in a specific angle. 

Adjust the Size of Salt Chlorinator

Many pool owners buy salt chlorinators that are too small for their pools. Many buy only 15 or 20-gram unit to keep the pool clean, which is ineffective (especially when the pool ages).

Make sure you invest in a salt chlorinator with proper capacity to keep your pool clean. For example, a typical 50,000-liter pool needs at least a 40-gram salt chlorinator unit.

Avoid Too Many Chlorine Tablets

Chlorine tablets contain stabilizers such as cyanuric acid. If you use this type of chlorine too often, the stabilizers will “lock” the chlorine, resulting in ineffective sanitizing action. Your water will turn green and full of algae even if the chlorine reading result shows a high amount of it.

Check and Replace Old Chlorinator Cells

Chlorinator cells can age, which reduce its function in cleaning the pool. A good chlorinator cell must produce robust hydrogen bubbles.

If the amount of bubbles looks diminished, the chlorinator cells may need replacements. 

Avoid Swimming with Dirty Body

Do you know why people must wash themselves before plunging into a pool? It is to reduce the amount of nitrogen in the water. Too much nitrogen will cause the chlorine to work extra hard in binding it, resulting in chloramines (combined chlorine). They are responsible for eye irritation, discoloration, and green water.

Keep the Pool Clean

This looks like an obvious suggestion, but many people neglect the cleanliness of their pools, until too late.

Avoid urinating in the pool, and make sure to not throwing anything into it, such as foods. Cover the pool when unused, during heavy rain, and when the sun is bright. During the winter, treat the pool with chlorine before and after the covering-up.

Regular maintenance is important to keep a swimming pool clean. Simple maintenance steps at home can also reduce the need to call an expensive professional when there are troubles.

So, Why is My Pool Green?

Green algae are the main cause of green pool water, but it is not the only one.

Yellow and black algae, mold, fungi, phosphates, debris, and bacteria can cause a discoloration in the pool. You need to conduct the right treatments to remove the green water and replace it with a clear, healthy one. 

Ask yourself: Why is my pool green? Are there any possible causes that I need to address? Understanding the possible causes helps you in finding the right products and solutions.

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