Oh No! Algae Bloom! Dealing With Green Pool Water

Last Updated: November 8 2016

Algae Blooms – they happen to all pool owners at some point. One day you go to bed with a sparkling oasis and wake up to a fetid swamp. If this is the first time you’ve experienced an algae bloom you may have many questions on your mind such as ‘how did this happen?’, ‘how do I fix it?’ and ‘will I need to drain my pool?’.

We’re here to answer all of those questions and more, so keep reading!

What is Algae?

Algae are water borne plants that grow in all types of bodies of waters; rivers, ponds, oceans and – you guessed it – your pool. The algae that grows in your pool is microscopic and can multiply very rapidly with up to millions of algal cells per millilitre.

Depending on its species, algae can purify waterways or suffocate and kill marine life. The algae in your pool is usually the harmless and staining type, but some species actually harbor e. coli bacteria. Best to remove it.

The types of algae you will be seeing in your pool are:

Green Algae

Yellow Algae

Black Algae

Did you know? Seaweed and kelp are both species of algae!

How Did it Happen?

Algae spores blow into your pool on the wind and thrive in warm, poorly circulated water – most often in the shade. Algae requires nutrients – specifically nitrogen, phosphorous and carbon to bloom.

What does this all mean? Pool water that is not adequately circulated and has unbalanced pH and alkalinity levels is a prime target for all sorts of algae blooms in the heat of summer.

How do I Fix It?

Of course, first you will need to determine which type of algae you have in your pool to treat it correctly.

When treating any form of algae it is important to balance the pH and alkalinity of your pool.

Green Algae is the most common type of algae you will find growing in your pool. It begins as large, green patches floating on the surface of the water, turning it murky and green and will also begin coating the sides of your pool making it slimy and disgusting. There are a couple of ways to combat green algae – and it’s not too difficult to get rid of completely.

One method is to shock your pool with chlorine. The ratio of chlorine is determined by both how much water your pool holds and how bad the infestation is. The amount of shock is either double, triple or quadruple times the recommended amount for the size of your pool. Light green water requires double shocking, dark green requires triple shocking and black green will require quadruple shocking.

Once this has been done, add an algaecide and let it sit over night. Vacuuming your pool will remove any dead algae cells that are floating around in the water.

You can also use a flocculant or ‘flocking agent’ sink floating algae particles to the bottom of your pool where you can them vacuum them up. After it has been added to your pool, allow it to circulate for a few hours, then leave it overnight.

When you vacuum your pool, you need to do it manually and set it to waste – you don’t want any surviving algae particles circulated back into your pool. You will often need to stop and allow the murk to settle before you can continue. You will likely lose a lot of water so you may want to leave the hose in or fill the pool to the brim before you start.

Yellow Algae – also known as mustard algae has a powdery look to it and it likes to grow in shady areas of your pool on the sides and floor. It’s more resistant to chlorine than green algae and needs to be approached differently. It also can infect your pool equipment and toys and bathing suits. Yellow algae is quite difficult to fight and keeping your pool free of it will be difficult.

It’s a good idea to turn the pump off in your pool so that the algae remains fairly stationary. Then you will need to scrub it from the walls and floors – that powdery film you see is a protective layer that is resistant to chlorine. Once you have thoroughly scrubbed all of the affected areas you will need to pour algaecide over them and leave it overnight. Much like cleaning up after using a flocculant you will need to vacuum manually and with your filter to waste.\

Black Algae is the most difficult to treat. Like the yellow algae, you will need to ensure that all pool toys and bathing suits are washed and disinfected; leaving your pool equipment in the pool will sanitize it along with the pool itself.

Black Algae puts down strong roots in plaster and concrete and you will need to scrub it with a stiff plastic – or even a steel brush. Triple shock your pool, let it circulate for 24 hours, then let it sit still for another 24 hours. Continue brushing the walls of your pool – the roots of the algae may still be embedded deep within the walls of your pool. Shock your pool again after a few days.

Vacuum up any debris on the floor of your pool with your filter on waste.

Keep your pool well-circulated and within recommended pH and alkalinity levels and wash all bathing suits before you or your family get in.


If you’re unsure or would like a second opinion we can come to you to test your pool. All products mentioned are available and our team will be happy to recommend the most effective one for your pool. Alternatively – Our friendly staff are also able to come out to test and treat your algae bloom for you!

Before & After

Green Algae


Black Algae