How to Kill Problem Algae in Pools and Spas

Well-balanced water and routine maintenance are the best way to enjoy your pool trouble-free. Keeping pH and sanitzers at the right levels are your basic defense against health and water appearance problems.

There are times, though, when there are unusual pressures on the water.

  • A lot of guests in a short space of time can add massively to the amount of organic material in the water.
  • A period of high winds can mean a lot of extra dust and plant material finding its way into the pool or spa.
  • In the fall, dead leaves can be a problem.
  • Even pollen can have a big impact at certain times of the year.

These things are important because algae is a plant and like any other plant, algae needs nutrients to grow. Ammonia in sweat and urine from extra guests provide a boost for plant growth. Dust and dirt add minerals the algae need and also block the action of sanitizers that might otherwise kill the algae.

The Cover Up Defense!

Pool and spa covers are the best defense against algae- keep out everything but water and people!. Regular use of the skimmer and vacuum removes potential nutrients. If you have a lot of kids regularly using a pool it will help if you make them shower before swimming.

Opening and Closing Swimming Pools

It is important not to close your pool in the fall until water temperatures have fallen below 60 degrees- especially if you use a mesh cover. Light can penetrate mesh covers but algae cannot grow below 60 degrees.

It is just as important to open your pool before temperatures rise above 60 degrees- or you might start the swimming season with a massive algae problem.

Types of Swimming Pool Algae

Green algae grow quickly in poorly maintained swimming pools.
Green algae grow quickly in poorly maintained swimming pools.

Green Algae

This is a quick growing plant. It will appear if the water isn't being properly filtered or if sanitizer levels are allowed to fall. The free floating variety will turn water green and cloudy. The filamentous form grows on surfaces in long strands.

Yellow Algae (or 'mustard' algae)

This can grow on the surfaces of the spa or pool and even form sheets of yellow slime if left unchecked. It is tougher to get rid of than green agae and once established, normal levels of sanitizer won't control it.

Black Algae

This is the worst of the algae from a maintenance perspective. It has a tough protective coat that stops chlorine sanitizers penetrating and "roots' that can invade porous or rough surfaces. Even if all the visible algae is removed it often grows back quickly.

'Pink Algae'

This is not an algae but a bacteria. It causes similar problems to green algae and needs similar treatment.

Spa and Swimming Pool Algae Treatments

Green algae can usually be dealt with by a standard chlorine or non-chlorine shock but the assault must be continuous on the first day. Ideally, you should test the water every hour or so to make sure the chlorine concentration is at shock levels. Your pool store should have test strips to measure high chlorine concentrations. On the following days, an hour or two cleaning surfaces should be enough for a small pool or spa.

If you are carrying out this treatment in lowish temperatures use liquid products or pre-dissolve solids as they will dissolve too slowly in cold water to be useful.

The First Thing to do is Get that Tub Clean!

Make sure you have a clean skimmer and basket. Clean the filter repeatedly during an algal attack because, as the algae die, they will clog the filter. Remember that sand filters can lose their 'edge' after a few years and may need recharging.

Cyanuric Acid Stabilizer

If you use a chlorine sanitizer in a pool then you probably use Cyanuric Acid stabilizer (CYA).This reduces the breakup of chlorine in sunlight but also affects how much free chlorine is available to attack the algae. If the stabilizer level is too high even very large doses of chlorine will not bring the free chlorine levels up to an effective concentration. On the other hand, some stabilizer is useful to maintain free chlorine levels in sunlight.

Try to get the concentration to between 30 and 80ppm before adding any chlorine. If the CYA level is very high it is worth replacing some pool water to bring it down.

Adjust pH

Make sure the water pH is about 7 ppm with test strips or reagents from your local or online supplier. The wrong pH means no chemicals work as they should.

Use the right Shock to clear up your Spa or Pool

A shock is a big dose of chemical cleaner. There are three types of chemical to destroy organic material and tackle algae and bacteria- chlorine, monopersulfate (MPS) and hydrogen peroxide. MPS is often simply called a non-chlorine shock. Chlorine shock is best for dealing with algae. It is compatible with either chlorine or bromine sanitizer systems but should never be used with biguanides like Baquacil or Soft Swim.

Chlorine Shocks (sometimes called super chlorination)

Check packaging on the chlorine product you use for the recommended doses.

You can also use a pool calculator to work out dose levels for shocking eg

Chlorine enhancers like 'Mustard Buster' or 'Yellow Treat' are good for boosting the attack on yellow algae.

Using Trichlor

(This is not recommended for vinyl swimming pools or people unsure of handling very concentrated chlorine solutions.)

Trichlor is the most concentrated form of chlorine and a good choice for tackling really tough algae, especially the black variety- but be careful how you handle this stuff, it can eat into skin and clothes! It is also a lung irritant.

One sixth of a pound of Trichlor per 500 gallons of pool or spa water will kill most algae quickly. Be aware that a solution this strong can discolour dark plaster spas and painted surfaces. Test a small area of any surface you are worried about with a concentrated chlorine solution before shocking.

Allow the super-chlorinated solution to circulate for about 3 days, brushing the pool or spa surfaces everyday. Check the filter and water pressure regularly.

Spot treatment of Stubborn Algae

For tiled pools or plaster-walled spas, tackle patches of tough algae with a stainless steel brush. Nylon-bristled brushes are best for any plastics or vinyl surfaces (ensuring the surface is not damaged) then apply concentrated chlorine solution directly to affected areas. Brushing is needed to break down the outer defenses of the algae and allow the chemicals to do their job.

Trichlor in a sock is an old pool professional's trick and can be used to scrub problem areas. Just remember that trichlor is a lung irritant so avoid breathing fumes. Also remember that any concentrated chlorine solution will attack natural products like cotton, wool or skin. Gloves are recommended!

Underwater spots can be treated by placing a trichlor tablet in a sock and hanging over the affected area. This will allow water to slowly dissolve the tablet and keep up a high concentration of chlorine where it is needed.

Clean Up After Treatment

Use a vacuum cleaner to remove dead algae and any silt-like materials form the bottom of the water.

Test to make sure chlorine has dropped to 3-5 ppm before bathing. If you use a bromine sanitizer start adding bromine when chlorine levels fall below 3 ppm.

Keep up a steady daily cleaning regime for a week or so and examine all surfaces for algae.The addition of a phosphate remover to the water can starve any remaining algae of nutrients.

If everything seems fine, you can return to your normal maintenance routine.

If there are any signs that the algae are regrowing, you can apply an algicide. Algicides are most effective when algae levels are low and will prevent regrowth or a return to serious problems.

If you still have problems with algae you need to ask yourself.

  • is the pool's circulation system adequate?
  • is the filter doing a good enough job?
  • am I really getting the water balanced?

For Bromine Sanitizer Systems

MPS (non-chlorine shock) can be used as a shock but is less effective than chlorine against yellow and black algae and not recommended.

For Biguanides (eg. Baquacil) Sanitizer Systems.

If you use a biguanide sanitizer follow the above program using hydrogen peroxide as a shock. Mixing biguanide with chlorine results in brown water discoloration and staining of pool, spa or hot tub surfaces. Using non-chlorine shock will destroy the biguanides.

Eradicate algae in swimming pools and spas.
Eradicate algae in swimming pools and spas.


Copper Sulfate

A very effective and cheap algicide but it will stain light colored plaster, wood and some acrylics, especially when chlorine is present. 'Black Zapper' is a copper sulfate based algicide used against black algae but be careful in use as high doses are toxic.

 Quaternary Ammonium compounds

Only really useful against green algal and then only before it gets a good grip on your pool or hot tub. Used as a preventative in some commercial pools.


A mixture of polymers and quaternary ammonium compounds.These are non-foaming algicides sold in concentrations of 30 to 60%. They are more effective, when dealing with yellow and black algae than ordinary quaternary ammonium compounds but more expensive. 'Polyquat 60' is a widely used preventative.

Non-Algicide Swimming Pool and Spa Algae fighters

Chlorine Enhancers that Tackle Yellow Algae

Mustard Buster, Yellow-out and Yellow Treat are commercial brands of chlorine enhancer that when combined with chlorine have a strong impact on yellow algae. Follow instructions on packaging.

Sodium Tetraborate

This is an effective, EPA registered, preventative against all forms of algae. It works by stopping the algae manufacturing the food they need. It has the additional advantages of clarifying and softening water. It also reduces eye and skin irritation.

'Proteam' is a manufacturer of sodium tetraborate


Algae bind to the surface of chitin particles and are removed by normal filtration. Pool clarifiers like Sea Klear are based on chitin. A useful and safe way to reduce algae build up but not effective if you already have a severe algae problem.

Ozone Generators

Ozone in the pool or hot tub water attacks all organic material and is a good way of controlling algae. It will also reduce the spa's consumption of sanitizers.


Copper ions from ionizers kill algae. The ions are produced by passing a low voltage current through a copper plate in the plumbing system. The ions are short-lived so you need good water circulation for maximum effect.

Making Sure the Algae Stays Away

As soon as you have eradicated all algae from the pool or hot tub it is important to establish a regular maintenance regime. Algae won't grow in well balanced water where levels of sanitizer, pH and minerals are kept within optimal ranges.

If you want an additional level of protection, a phosphate remover like PhosFree offers a good insurance. Algae can't grow without phosphorus in the water.

Always use phosphate-free cleaners recommended for spas, hot tubs or swimming pools.

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Comments 5 comments

Nick 4 months ago

These algae are tough. Thanks for sharing these very informative and helpful article. I would like to know, what should be done to avoid these algae before its too late?

kubth profile image

kubth 6 years ago from UK

Agree with the external commenters, great hub about pool and spa algae. We generally use and recommend non-chlorine shock.

pool service jacksonville 6 years ago

Good article, I thought these problems could only be resolved with chlorine, thanks for these alternatives.

EZ Swimmer 6 years ago

The ph level is key. Very well written Hub! This is hard information to convey. Excellent job!

Pool Filters 6 years ago

Great hub! This is by far the most comprehensive info on killing algae that I have been able to find on hubpages! Thanks for sharing!

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