Algae in swimming pools

8th Oct 2017

Algae in swimming pools

Algae in a Swimming Pool

Algae in a swimming pool is not fun. There are numerous effects to health that occurs when algae which is a type of mold is breathed in. It also can cause a rash, headaches and your pool just looks terrible when infected with algae.

Naturally the best way to deal with a problem is to see to it that it doesn’t occur in the first place! If you suspect you have algae in your pool the first step is to identify it.

Identification of Algae

There are a few ways to determine if algae is present in your pool. You may notice the water looks cloudy, there may be obvious spots and you may see an increase in the amount of chlorine you require.

Once you suspect that algae is present, the next step is to figure out what type of algae. There are a number of different types of algae with some more sinister than others.

Black Algae (blue green algae)

Likes to inhabit fine cracks ad crevice’s in the pools surfaces pool surfaces, plaster gunite and pebble crete surfaces are especially prone to fall victim to this problem.

Black algae grow more easily in the shady areas of the pool but can be found in the sunny areas as well to a lesser degree It is very unusual to find it in vinyl as these don’t have any cracks for the roots to get a hold.

Black algae has a heavy waxy or slime layer and is able to grow over the top of itself which protects it from chemical attack very effectively unless this layer is damaged through scrubbing and then exposed to high levels of chlorine that are around 2.5 ppm

With black algae the pools water stays clear but in nearly all cases the demand for chlorine increases considerably.

Black algae can also be confused with other types of stain however if it brushes away it is algae and something must be done about it as quickly as possible.

Note if black algae is not completely eradicated i.e. roots and all the roots will regenerate algael growth It is imperative that the visible algae be brushed away which will expose the roots to the effects of shocking and algaecide.

Black algae needs to be thoroughly brushed with a suitable brush off the affected area every day until it is totally gone. Algae must not be allowed to cover the roots in the cracks thus protecting them from chemical attack.

Green algae

Green algae vary in colour from blue green to yellow green to dark green.

It can be free floating in the water turning it cloudy green. It can form on the walls of the pool in green patches or even cover the whole pool in severe cases.

Green algae is able to clog filters and may cause surface damage if it is not dealt with. It is easily treated with shocking & algaecide.

Small celled green algae (SCGA)

This type of algae is different from green algae in that the water stays relatively clear.

It is often confused with a copper or mineral problem; however metal chelating agents will be ineffective.

When treating with chlorine, chlorine seems to "disappear". SCGA is very resistant to even high levels of chlorine.

Other green algae noticed as green spots all over the swimming pool, particularly in shady areas. The water is almost nearly always very clear. The water may cause a stinging sensation. This is normal green algae, typically brought about by lack pool maintenance.

Mustard algae

A chlorine resistant form of green algae (yellow green to brown in color)

Often found in sunny temperate areas and can resembles dirt or sand on the bottom and sides of a pool.

Mustard algae is easily transferred to infect other pools in swimming gear that is not cleaned between uses. So take particular care after having used a public pool that swim gear is cleaned before introducing it to your own pool, but naturally that is an important rule to adhere to in all cases where swim gear is used in areas other than the home pool.

Mustard Algae can be brushed easily, however it tends to return quickly to

almost the exact same location. This makes it difficult to maintain a steady chlorine reading or residual (1.0 -2.5 ppm), it can at times survive in high levels of chlorine, and is not necessarily slimy. The water tends to be otherwise clear. It can also be confused for copper or iron stains however if you can brush it away it will be algae.

It is very important that mustard algae growth be removed from your pool and that all equipment used in the pool be brushed down and cleaned. This includes hidden areas like the back of underwater lights and ladders. Bathing suits should also be washed after each swim to avoid contaminating other pools. Any equipment used in the contaminated pool should be sanitised before being used again in any pool even if it has been dry.


From time to time there is just no avoiding having to deal with algae contamination

The treatment of algae contamination must be aggressive and all of it must go or else will have to start all over again within a few weeks. Here is a general summary for the treatment of algae.

Clean all the surfaces of the pool paying particular attention to the shady spots.

Al effected surfaces must be scrubbed down vigorously.

Put your first dose of algaecide into the water then shock the pool with a powerful dose of chlorine or whatever sanitizer your pool uses, at this stage it may be wise to sample the water and get some additional advice about what you need to do in your particular situation

Run the pool straight for 24 hours a day or until it clears

Take a sample of the water for testing after about 4 or 5 days and see if your water is back to normal

Make sure you have cleaned and sanitised all of the vacuum hoses brushes and other pool equipment and toys that you have used before using them again.

Preventing an algae bloom in your swimming pool

First of all, keep in mind that aside from coming from the air algae can be introduced to the pool by things such as swimming costumes that have been left unwashed after having been worn in another pool, a river or even the beach. Toys that were used in alternate locations such as floatation devises paddling boards etc should also be well cleaned or have spent a reasonable amount of time out in the sun before being introduced into the pool.

Regularly brush the pool down on a weekly basis and pay attention to those parts of the pool that spend the most time in the shade as that is usually where algae become visible first. Pay attention to fitting such as steps around water inlets and other pool fittings where shady spots can happen. Take for example the area over the top of the vacuum plate which is constantly in shade. Leave it exposed to the sun from time to time but be careful that no one steps in it and hurts themselves by putting some barrier or notification near it warning of the danger.

Maintain the waters balance and make a point of checking it after heavy use, after rain or after the pools level has been toped up with fresh water. If you don’t have the equipment to do this yourself you will find that most swimming pool shops will do it free of charge for you and advise you as to what is required to restore it to the right balance

If you use cartridge filters to clean them with an approved chemical cleaner every six weeks or so.

Regularly shock and add algaecides as required depending on the season, location and use

Keep the equipment that you clean the pool with clean by washing them with fresh water, this is particularly important if you recently used it to clean an algaecide contaminated pool. As they could reintroduce algae back into the water.

Leaving your pool equipment exposed to the sun is a great way to kill of unwanted organisms on them however keep in mind that there is a lot of pool equipment that is just not up to over exposure to UV radiation.

Run the filter regularly depending on the climate and use and keeping in mind that if you have a salt water chlorine pool you need to run it for a minimum of time to generate enough chlorine to effectively disinfect the water. The amount of time required is something that can be learned by reducing the filtration time while ensuring the correct water balance.

Remember that preventing an algae bloom in your pool will save you much time and effort down the track. If you keep a regular swimming pool maintenance program you will decrease the chances of an algae outbreak. If your swimming pool does become infected, follow the guidelines and your swimming pool will be back to pristine in no time.

For other pool maintainence ideas you may be interested in these articles:

Maintaining the PH of a swimming pool

Using an inline leaf canister with an automatic pool cleaner

Checking the water level of your swimming pool

The Dragonfly Floating Pool Cleaner - a way to clean your pools surface BEFORE the debris sinks