How to Treat Algae in a Swimming Pool

Co-authored by wikiHow Staff

Last Updated: October 21, 2022 References

If you want to keep the water in your pool looking clean and clear, you'll occasionally have to remove the algae that can cause your pool to look green, yellow, or black. We'll help you tackle all the different types of algae that could be affecting your water!

Part 1

Part 1 of 3:

Getting Rid of Green Algae

1
Test and adjust the pool's pH. One of the main causes of algae growing in a pool is if the pH of the water gets too high, because this prevents the chlorine from killing the algae. Collect some water from the pool and test the pH levels with a test kit.
- To lower the pH, add some muriatic acid or sodium bisulphate. To increase the pH, add sodium carbonate.^{[1]
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- The ideal pH for pool water is between 7.2 and 7.6.
2
Shock the pool. The best way to get rid of green algae is with a combination of shocking and algaecide, which is why it's so important to balance the pH level of the water first. The intensity of the shock will depend on how much algae there is:^{[2]
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- For light green algae, double shock the pool by adding 2 pounds (907 g) of shock per 10,000 gallons (37,854 L) of water
- For dark green algae, triple shock the pool by adding 3 pounds (1.36 kg) of shock per 10,000 gallons (37,854 L) of water
- For black-green algae, quadruple shock the pool by adding 4 pounds (1.81 kg) of shock per 10,000 gallons (37,854 L) of water
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3
Add an algaecide. Once you've shocked the pool, follow up by adding an algaecide. Make sure the algaecide you use has at least 30 percent of the active ingredient. Follow the manufacturer's directions to determine how much to add based on the size of your pool. After adding the algaecide, let it sit for about 24 hours.
- An ammonia-based algaecide will be cheaper and should work with a basic green algae bloom.
- Copper-based algaecides are more expensive, but they're also more effective, especially if you have other types of algae in your pool as well.^{[3]
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  Research source} Copper based algaecides do tend to cause staining in some pools and are the main cause of “green hair” when using a pool.
4
Brush the pool. After 24 hours with algaecide in the pool, the water should be nice and clear again. To make sure you remove all the dead algae from the sides and bottom of the pool, brush the entire surface of the pool.
- Brush slowly and thoroughly to ensure you cover every inch of the pool's surface. This will prevent the algae from blooming again.
5
Vacuum the pool. Once all the algae are dead and have been brushed off the surface of the pool, you can vacuum them out of the water. Be slow and methodical when you vacuum, making sure you remove all the dead algae from the pool.
- Set the filter to the waste setting if you're using it to vacuum the pool.^{[4]
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6
Clean and backwash the filter. Algae can hide in a number of places in your pool, including the filter. To prevent another bloom, clean and backwash the filter to remove any leftover algae. Wash the cartridge to dislodge any algae, and backwash the filter:^{[5]
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- Turn off the pump and turn the valve to “backwash”
- Turn the pump on and run the filter until the water runs clear
- Turn the pump off and set it to “rinse”
- Run the pump for a minute
- Turn the pump off and return the filter to its normal setting
- Turn the pump back on

Part 2

Part 2 of 3:

Treating Mustard and Black Algae

1
Test and adjust the water pH. When the pH of pool water gets too high, it prevents chlorine from being effective, meaning it can't prevent or treat an algae bloom. Add pool water to a test kit and test the pH of the water.
- Lower the pH by adding muriatic acid or sodium bisulphate. Increase the pH by adding sodium carbonate.^{[6]
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- Test the water again after treating it. The ideal pH is between 7.2 and 7.6.
2
Brush the sides and floor of the pool. Black algae especially are very resistant to shocking and algaecide, so you must brush the algae before treating the pool. Brushing breaks up the protective slime layer that grows on the outside of the algae.
- Brush the bottom and sides of the pool thoroughly and vigorously to break through the algae's protective layer.^{[7]
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3
Shock the pool. Like green algae, the best course of action to treat mustard and black algae is to add shock and algaecide. Immediately after brushing, triple shock the pool by adding 3 pounds (1.36 kg) of shock per 10,000 gallons (37,854 L) of water.
- During the shocking and algaecide treatments, leave the pump and filter running all the time.^{[8]
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4
Add algaecide. For mustard and black algae, it's important to use copper-based algaecides. Ammonia-based ones will not be effective against these types of algae. Use an algaecide with at least 30 percent active ingredient, and you may want to go as high as 60 percent.^{[9]
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- After adding the algaecide, let it sit overnight.
5
Brush daily for a week. Continue brushing the entire surface of the pool every day for the next seven days. This will make sure the algae's protective layer gets penetrated with shock and algaecide, which will ensure the treatment is effective.^{[10]
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6
Shock the pool again. After three to four days of daily brushing, shock the pool a second time with a regular shock quantity. Add 1 pound (454 g) of shock per 10,000 gallons (37,854 L) of water. This will guarantee the chlorine has enough strength to kill the algae.^{[11]
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- After shocking the pool again, continue brushing daily for the next three or four days, or until the algae is gone.
7
Vacuum and clean the filter. Once the algae are dead, vacuum the pool thoroughly to remove all the dead plant matter. When the pool has been vacuumed and the water is clear, clean the filter. Rinse the cartridge, backwash the filter, and clean the filter with filter cleaner.

Part 3

Part 3 of 3:

Preventing Algae from Returning

1
Wash all your bathing suits. Algae can actually hide away and grow on things like swim suits and towels. To prevent your clothes from re-infecting the pool, wash all your bathing suits and towels in a washing machine.
- When the garments and towels are dry, dry them in the dryer to kill any leftover algae.^{[12]
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2
Sanitize pool equipment and toys. Wipe down all your toys, floats, and cleaning equipment with a non-bleach disinfectant. This will kill any algae that's hiding out on these items, and prevent the algae from blooming again. Items to clean include:^{[13]
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- Vacuum
- Brushes
- Noodles
- Floats
- Balls and water sport equipment
3
Maintain the right chemical balance. Test and adjust your pool's levels on a regular basis to maintain the right levels. Chlorine, pH, and alkalinity should be tested twice a week during pool season.^{[14]
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- The ideal pool pH is between 7.2 and 7.6
- The ideal chlorine level is between 1.0 and 2.0 parts per million
- The ideal alkalinity is between 80 and 120 parts per million
4
Brush, vacuum, and filter the pool regularly. A well-maintained pool with good filtration and circulation is much less likely to get algae than one that's not cleaned regularly, that isn't filtered properly, and that doesn't have good water circulation.^{[15]
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- The pump and filter should run between eight and 12 hours a day during pool season.
- Brush and vacuum the pool every two days.
- Clean and sanitize pool equipment and toys every one to two weeks.

Community Q&A

Question

Will a solar cover cause more algae to form?

Community Answer

It depends how long you leave the cover on and if you have the pump running. Heat dissipates chlorine and algae grows in dark areas, so have the pump running and only leave the cover on 8-10 hours a day and you should be fine.