How to Get Rid of Blackspot Algae in Your Swimming Pool

Co-authored by wikiHow Staff

Last Updated: January 29, 2023 References Approved

Algae are diverse, simple plants that thrive in water. Of all the alga types and species on the planet, black algae can be the most difficult to treat and most resilient to remove from a swimming pool. Alga spores may always be present in your pool, but problems arise when the right conditions are present and the algae are able to bloom. There are ways to treat your pool and rid it of black algae, but prevention is always the best approach.

Part 1

Part 1 of 2:

Preventing Black Algae

1
Understand black algae. Like any alga species, black algae (which are really blue-green algae that look black) will grow quickly in warm water on a sunny day. Black algae are chlorine-resistant, because they have a protective outer layer that makes regular sanitization ineffective. While algae aren’t harmful, their presence can be a sign that other bacteria are present.^{[1]
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- Other alga species that are common in pools are green algae, mustard or yellow algae, and pink algae, which is actually a bacterium.
- Algae tend to bloom in pools that have poor circulation, poor filtration, a high pH level, and a low chlorine level.^{[2]
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2
Test pool levels and balance chemicals often. Chlorine should be somewhere between 1.0 and 3.0 parts per million. The pH level should be between 7.2 and 7.8. Alkalinity should be between 80 and 120 parts per million. Test these three levels once or twice a week and correct as needed.^{[3]
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- Calcium hardness should be tested monthly, and the ideal level is between 180 and 220 parts per million.
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3
Shock the pool regularly. On a weekly or bi-weekly basis, shock your pool to remove chloramines (chlorine molecules that have attached to other particles) and raise chlorine levels quickly. There are four products you can use to shock your pool, and they are:^{[4]
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- Calcium or lithium hypochlorite
- Granular chlorine
- Potassium peroxymonosulfate
4
Circulate the water. Keeping the water in your pool moving is one of the best preventative measures for algae. Not only will it make it harder for alga spores to take hold, it will also ensure that sanitizing chemicals are evenly distributed. Monitor and clean the skimmer and strainer basket regularly.^{[5]
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5
Clean your filter. A clogged filter won’t work as well, and it will slow down the flow of water. In warmer months like July and August when algae have the best chance of blooming, run the pump for about eight hours over the course of the day.
6
Consider using a preventative algaecide. Especially if you’ve had trouble with algae in the past, consider adding algaecide to your pool on a monthly basis.^{[6]
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Research source} Ammonia-based algaecides should work for green algae, but metal-based (specifically copper) algaecides are necessary for black algae.^{[7]
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7
Brush and vacuum your pool on a weekly basis. Brushing will remove dirt and spores, and prevent algae from taking hold.^{[8]
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Research source} Brush the walls, floor, and steps of the pool. Once you’ve completed your brushing, vacuum the pool to dislodge and remove any spores that have settled on the bottom.
- Use a steel brush for concrete or plaster pools. Use a nylon brush for vinyl, fiberglass, acrylic, or painted pools.^{[9]
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Part 2

Part 2 of 2:

Treating Black Algae

1
Brush the pool. Brush daily for a week with the proper brush for your pool, paying special attention to algae-affected areas. The protective layer that forms on black algae makes the algae otherwise impervious to regular sanitizers, so you must brush the algae to break through this layer so that chlorine and algaecides can kill the organism.^{[10]
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- Black algae have deep roots that work their way into the walls, grout, and filters in your pool. If the roots aren’t destroyed, the organism will continue to grow back.
- Try rubbing the algae spots with a chlorine tablet after brushing.
2
Add algaecide to the water. After brushing the pool for the first time, add a copper-based algaecide that has an active ingredient concentration of at least 30 percent.^{[11]
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3
Shock the pool. After the algaecide has run its course, shock the pool. Continue your daily brushing, and after three days, shock the pool again.^{[13]
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4
Vacuum the pool. This will remove the dead algae and any debris that has settled on the bottom of the pool. To remove larger quantities of deal algae, consider adding an alum flocculant, which will cause the material to clump together for easier removal.^{[14]
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5
Clean and sanitize filters, equipment, and toys. Black algae can not only clog the filtration system and make it less efficient, but spores can also hide in the filters and bloom again when you least expect it. Wash all bathing suits and towels in the washing machine, and dry them in the dryer to kill any algae that may have attached to the fabric. Sanitize pool equipment and toys by scrubbing them with a bleach-based cleanser.^{[15]
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6
Test the water. You should do this regularly, but especially if you have recently adjusted the chemical levels of the pool.

Community Q&A
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Question

Can algae in a pool hurt you?

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This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness.

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Staff Answer

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Algae itself isn't harmful to humans, but you still shouldn't swim in a pool with algae! This is because if algae can grow then harmful bacteria and other microbes are likely living in your pool as well and could cause a serious illness or infection.
Question

Is silver-based algaecide better than copper-based for treating black mold?

Community Answer

Copper has ions that kill plant life; silver doesn't. While silver can destroy vegetation, copper naturally deters and inhibits growth.
Question

Is black algae harmful to swim in if detected in pool before it gets treated?

Community Answer

No.

Warnings

Use copper-based algaecides (and all pool chemicals) according to the manufacturer’s directions. High copper levels can cause staining, and can even turn blonde hair green. Handle all chemicals with caution.

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