Why is my swimming pool green? (how to easily fix it)

swimming pool green

If you’re lucky enough to have an outdoor swimming pool in your garden, you’ll know it can actually be a curse in disguise. You probably have it covered during the winter to protect it from the elements.


At the start of summer, you pull the cover off, only to discover the water has turned green. You wonder, where’s the calm blue water?!


The bad news doesn’t stop there, however. It will take time and effort to kill the green and restore the blue water. As part of swimming pool cleaning basics, it’s something you need to know.



Causes of swimming pool green water

So, why is my swimming pool green? Algae!


But that’s just half the story. Having a small number of algae alone won’t turn a pool fully green. However, having a chemical imbalance and a warm climate will help algae to grow and spread. And at an exponential rate.


While algae aren’t harmful by themselves, it does make for an uncomfortable swimming experience. Also, discoloured water can enable other pathogens in the form and spread. Some of these can be harmful. Hence why if you have swimming pool green water, you need to treat it.



Preventing algae from growing

It’s fairly straight forward to prevent algae from growing in a swimming pool. But you’ll need to keep on top of it to stop algae from ever returning!


Check chemical balances

First, you’ll need to balance pH and ensure you have the correct Chlorine levels. Should the pH be incorrect then the Chlorine has reduced effectiveness, and so this creates the conditions that Algae love!


If the pH level is too low, the water is acidic and over time, it will erode plastic and metal components. Too high and bacteria won’t be effectively killed and also there will be a build-up of calcium and other minerals.


We recommend using Clearwater CH0012 Dip Test Strips (Available for £8 at Amazon) to test pH levels. A healthy pool is between 7.2 and 7.8 on the pH scale.


Clean the filters

Second, check your filters as not all water colour issues are the fault of the water itself. If your filter system isn’t working properly then it unlikely to catch and remove the algae and other debris on the surface. When a pool’s water is left untreated, it can become a breeding ground for algae and other fungi which will, in time change the colour of the water.


Thankfully, it’s easy to clean to pool filters and they should be disassembled and thoroughly cleaned regularly.



pool water pH test kit



Pool chemistry!

So once we’ve cleaned the filter and tested the pH level, it’s now time to balance the pool chemistry. You should balance the pH before treating the pool. Either add an acid or a base to bring the pH level to just around 7.8.


While this is at the high end of the range you would normally want in your pool, it’s necessary when you’re treating it for algae.


Firstly, turn on your pump so the chemicals will circulate throughout the pool.


Then correct the pH level by either increasing the pH with sodium carbonate or decreasing it with sodium bisulfate.



Get scrubbing!

With the pH balanced, we still have algae to get rid of. In order to maximize the chemical effectiveness, we need to break up large clumps of algae. So it’s time to get scrubbing!


If you have a vinyl pool, use a nylon scrub brush. Wire brushes might damage vinyl pools but are safe to use on plaster pools.



Time to Shock!

After balancing and scrubbing, then it’s time to use a shock treatment. While this is only ever a temporary solution, it’s the best way to quickly remove excessive levels of algae and other build-ups which are keeping the water green or discoloured.


A shock treatment will add a large dose of chlorine and algaecide to the pool. While there’s a number of options, we recommend using Happy Hot Tubs Chlorine Shock Pot (1kg costs £15 at Amazon).


Once added, allow the pool to sit for 24 hours to kill off the algae and bacteria. After 24 hours, you can clean the pool and test and re-balance the pH levels.


It’s then worth using an algaecide such as the Clearwater CH0006 (1-litre costs £9 at Amazon) as this will help shift the last of the algae and prevent future build up.



pool cleaning



Keep your swimming pool green free

Keeping your swimming pool green water free, requires regular maintenance. It’s worth checking pH levels weekly and cleaning filters every few weeks.


If course, you will want to add more chlorine as and when necessary. Adding algaecides is also a good idea and you can add some weekly whether you add chlorine or not.


It’s worth removing debris before using the pool and scrubbing the walls and vacuuming them as well as the bottom each week.


It’s a lot of a lot of work to successfully maintain a swimming pool. But it’s well worth it as a well-maintained pool will provide more enjoyment.



Pool maintenance tips

  • Make sure you have a reliable chlorinating system, whether it is an in-line, floater, or salt system. Chlorine needs to be in the pool always. Throwing a jug of liquid in it once a week isn’t a good way of maintaining the pool. A simple tablet chlorine floater is very effective.
  • Use a water clarification solution.
  • Clean your filter.
  • DE filters: Backwash once a month.
  • Sand filters: Backwash once every two weeks. Be sure to backwash your sand filter for a minimum of four minutes; otherwise, you will see filthy water shooting back into the pool.
  • Cartridge filters: Clean it every three to four weeks, unless you see algae in the pool, in which case you should clean more often. Here’s more about how to clean a cartridge filter. Soak in trisodium phosphate every three months.