One of the most disappointing and frustrating things for pool owners to discover, when they get up in the morning, is green pool water. Yesterday the pool water was sparkling clear and now it’s a disgusting green or dark mustard color. Find out what went wrong and how to clear green pool water.
If you want your family and friends to safely swim in a crystal clear pool with properly balanced pool water you need to understand basic pool chemistry. All it takes is a little research but don’t worry I have done that for you!
Usually swimming pool water has turned green because algae has begun growing in your pool. And algae can only grow and flourish when the free chlorine in your pool has fallen below the normal level.
So we need to increase the free chlorine level in your pool.
Table of Contents
Why is My Swimming Pool Water Green?
The most common reason for pool water to turn green is the growth of algae in the water. Algae spores are commonly found everywhere in nature and will be blown by the wind into your pool. These spores can attach themselves to pool cleaning equipment, pool toys and your swimwear. In fact, it is virtually impossible to eliminate algae spores from your pool environment.
When the free chlorine level in you pool water drops below the normal level for effective sanitization algae spores will begin to replicate causing an algae bloom in the water.
Even if you fill your bath with water and leave it for a few days it will start to turn green because there is no chlorine to prevent the growth of algae and bacteria.
However, algae blooms can be triggered by other imbalances such as incorrect pH, hot weather, heavy rain, dirt and nutrients in the pool water.
Can I Swim in Green Pool Water?
No, it’s unhealthy to swim in green pool water. The water has become saturated with green algae such as Cyanobacteria which produces cyanotoxins that are dangerous and can cause sickness in all animals and humans.
What is Algae?
Algae is the informal term that commonly refers to the large and diverse grouping of unicellular bacteria (such as Chlorella and Cyanobacteria) and other micro-plants. There are literally hundreds of different species of algae-type micro-plants and unicellular microbes. The most common type of algae is green or yellow in color because they contain chlorophyll to photosynthesize light into energy and oxygen. These algae can rapidly reproduce from asexual cell division to take over your swimming pool water in a matter of days.
It is important to take action immediately because if left untreated your pool will look like a Florida swamp within a few days. Algae can grow at very rapid rate quickly taking over your pool water and even clogging the filtration system.
Untreated algae blooms only get worse and can cause black spots on the pool surfaces which are difficult to remove. The algae can take root into the walls of the pool causing micro-cracks and pitting of the surface. Algae will also grow in the pool plumbing and filter cartridge causing reduced water flow and less efficient filtration of the water.
How to Clear Green Pool Water
Follow these easy steps to learn how to clear green pool water.
Are My Pool Chemicals in Balance?
The first thing to check is the chemistry of your pool water using a pool test kit or chemical test strips. If the water is out of balance you need to correct the issue before you can successfully eliminate algae from the pool.
Clean the Sides of the Pool
If there is algae in your pool it will stick to the sides and bottom. Simply treating the pool water with extra chlorine will not necessarily remove algae from the pool surfaces. You will need to brush the pool surfaces to mobilize the algae into the water so that the filtration system can remove it from the water.
Vacuum the Pool
After brushing the walls of the pool the algae will settle on the bottom. The next thing to do is to manually vacuum the bottom of the pool but switch the water flow to the stormwater waste on your pump so you are vacuuming the algae into the drain. Otherwise, you will be clogging your filter with algae and you will need to clean the filter cartridge or backwash the filter unit to the drain. If your pool water level is too low after vacuuming add more water to the pool.
Boost Chlorine Production
Salt water pools have a chlorinator cell installed after the filter to generate chlorine in the water before the filtered water is pumped back into the pool. The chlorinator control unit on the wall controls the flow of electricity through the salt water chlorinator cell to release free chlorine (from the salt) which sanitizes the pool. However, the amount of chlorine generated from a salt water chlorinator cell is only designed to be enough to maintain an already clean and clear pool.
Most chlorinator units allow the user to adjust the production of chlorine from the chlorinator cell. This option is useful to increase chlorine production during the summer when chlorine evaporates faster, after heavy pool usage, after rain has diluted the pool water or additional water was added to compensate for evaporation.
Simply dialing up the production of chlorine will not normally be enough to overcome an algae problem. Also you can run the pump for a longer period of time to produce more chlorine but again this is not normally going to produce enough free chlorine to eliminate an algae bloom.
To eliminate an algae bloom you need to “shock” the pool water with a high dose of free chlorine. This kills the algae and clears the green pool water.
How to Shock the Pool Water
The term “shocking your pool” means super chlorinating the pool water to a high dose of free chlorine that it is strong enough to kill algae and bacteria. This is achieved by adding liquid chlorine or a granular dose of calcium hypochlorite shock treatment.
If the water is pale green a standard dose of shock treatment will be sufficient to clear up the problem.
However, the darker the pool water the more shock treatment will be required.
Medium green water requires about double dosage of shock treatment.
Darker green water needs a triple dosage.
Black spots and pink slime will require four times the standard dose of shock treatment.
It may take several days and multiple treatments, depending on the severity of the algae bloom, for the algae to die and the filter to clear up the pool water.
Run the filter until your pool water becomes clear. You may want to run it overnight. You will need to clean the filter during this process as it will become clogged with dead algae. Depending on your filter system, either backwash the filter or remove the filter cartridge and hose out the debris.
If your pool water is still cloudy the next day you can add a pool water clarifier chemical which helps to combine the small suspended particles in the water into larger particles that the filter can remove.
Once you pool water is clear again you can get back to a normal free chlorine level by returning to your routine maintenance and/or salt water chlorinator setting.
Test the water again and adjust your water chemistry if necessary.
Read our post on How to Shock a Swimming Pool
How Often Do I Shock the Pool?
During the summer months it is recommended to shock your pool with a dose of shock treatment once a fortnight to ensure the level of free chlorine is maintained to a level where algae spores cannot reproduce.
Drain the Pool to Clear Green Pool Water
If the algal bloom is severe causing the pool water to become dark green and you are unable to see the steps in the pool it’s probably better to drain the pool, thoroughly clean it out with an acid wash and refill with fresh water rather than spending money on lots of pool chemicals.
How to Prevent Green Pool Water
Add Algaecide to the Pool Water
Add a dose of algaecide to the pool water at the beginning of the summer period or when you open your pool for the year. This will kill algae before it can have the opportunity to grow and bloom.
Test Phosphates in Your Pool
High phosphate levels are an indication of excessive organic material in your pool water. This is often the primary cause of algae growth, as algae is a water plant that loves to thrive on phosphate.
Common pool test kits and chemical strips don’t test for phosphates. However, you can purchase pool phosphate test kits and strips which are inexpensive.
Ideally the concentration of phosphates in a pool should be below 250ppb (0.25ppm).
Please read our article on How to Remove Phosphates in Your Pool
Is My Pool Pump Working Properly?
Your pool filtration system may be running normally but have you carried out the normal pool maintenance schedule and regularly cleaned the filter unit?
When the filter becomes clogged with debris the pump pressure increases and the water flow rate to the pump decreases causing the filter to become less effective at cleaning the pool water. It is essential to regularly clean the filter as per your normal maintenance schedule otherwise you run the risk of tipping the balance toward your pool water becoming unclean and unsanitized.
New Salt Water Chlorinator
If your pool is salt water chlorinated check the age and condition of the chlorinator cell. The average salt water chlorinator cell lasts between 3 to 4 years. Eventually the electrodes in the cell wear out over time. When the cell is old its effectiveness is reduced and produces less chlorine. This can sneak up on you to cause a gradual reduction in the production of chlorine. This causes the free chlorine level in your pool to suddenly fall away over a few days leaving the pool completely unchlorinated and vulnerable to attach by algae and bacteria.
Replacing the cell is a simple process and gives immediate results.
New Filter Cartridge
Depending on usage, filter cartridges have a typical lifespan of 2 or 3 years. Eventually they become permanently soiled and lose efficiency. If your cartridge is old and still looks dirty after you clean it then it’s probably time to replace it. When you replace the filter cartridge you will notice that your pool water looks sparkling and much cleaner.
Can My Pool Pump Handle the Size of My Pool?
If you have correctly balanced pool water and you have regularly cleaned the filtration system but are still experiencing multiple episodes of algae blooms, particularly in the summer months, your pool pump may not be handling the required workload to keep your pool sanitized with a sufficient level of free chlorine.
Check that the water circulation is strongly flowing around the pool when the pump is operating. If there is low pressure of water circulation the filter and salt water chlorinator (if installed) will not function effectively. this will leave your pool water less sanitized and open to the possibility of an algae bloom.
Are you finding that you need to increasing run your pump for longer periods of time? This could indicate that your pump isn’t handling the workload. You would be better off with a new, more powerful pump. A new pump is often the best solution as it would be running for shorter periods of time saving you electricity. It would help clean your pool more effectively and new pool pumps are less noisy than the old models.
Cover the Pool
When you are not using your pool install a pool cover. This minimizes algae spores, dirt, dust and other debris from entering your pool. Also the cover will prevent sunlight reaching the water stopping the growth of algae.
Other Ways to Prevent Algae
Ask swimmers to take a quick shower before entering the pool. This limits contaminants such as sweat, shampoo, skin moisturizer or sunscreen from entering the pool. These contaminants can deplete the level of free chlorine in the water.
Rinse your swimwear in fresh water after use to remove algae spores.
Regularly clean pool equipment and toys with soapy water or a mild cleaning product. This removes algae spores and other contaminates.
Follow your weekly, monthly and yearly maintenance schedule. This ensures your water chemistry is balanced and the filtration system is operating effectively.
Conclusion
How to clear green swimming pool water is a very common problem among pool owners. But it doesn’t have to be your problem! When you follow a basic pool maintenance plan, test your pool water regularly and balance your pool water chemistry you should never have to encounter pool water that has turned green.
I hope you found this information useful. If so, please share this web page with other pool owners so everyone can enjoy a problem free pool lifestyle.