Green pool water usually indicates algae growth caused by low sanitizer levels, poor circulation or a combination of water-balance problems. It can develop after heavy rain, hot weather, an equipment problem or a period when the pool has not been maintained.
The recovery process takes more than adding a large amount of chlorine and hoping for the best. You need to identify the likely cause, correct the water chemistry, remove the algae and keep the filtration system working throughout the cleanup.
Important: Do not swim in a green or cloudy pool. You should be able to see the bottom clearly, and the water chemistry must be back within its normal range before anyone enters. Always follow product labels and never mix pool chemicals.
Is the Green Colour Caused by Algae?
Cloudy green water and slippery pool surfaces usually indicate algae. However, clear water with a green tint can sometimes be caused by metals such as copper or iron.
Adding more chlorine to a pool affected by metals may intensify the colour. If the water is clear rather than cloudy, have a pool-water sample tested before beginning an algae treatment.
Test the Pool Water
Start with a complete water test. At minimum, check:
- Free chlorine
- Combined chlorine
- pH
- Total alkalinity
- Cyanuric acid or stabilizer
- Calcium hardness
- Salt level, if you have a saltwater pool
A professional water test can provide more reliable results when the pool is severely unbalanced or the cause of the colour is unclear.
The pH level affects how efficiently chlorine works. Correct major water-balance problems before shocking the pool, following the recommendations from your test results and the instructions for your pool surface and treatment products.
Check the Pump and Filtration System
Chemical treatment will not clear the pool if the water is not circulating properly.
Before proceeding, confirm that:
- The pump is operating
- Water is returning to the pool
- The skimmer and pump baskets are clear
- The filter pressure is within its expected range
- Valves are in the correct positions
- The water level is high enough for proper circulation
Weak flow, unusual noises, leaks or rapidly changing filter pressure may indicate an equipment problem that needs to be corrected first.
Remove Leaves and Debris
Use a skimmer net or leaf rake to remove as much debris as possible. Leaves, dirt and other organic material consume chlorine and make the cleanup process harder.
If the bottom cannot be seen, work carefully and avoid using equipment that could damage the pool surface. Removing large debris before treatment gives the sanitizer a better chance to address the algae.
Brush the Pool Surfaces
Brush the walls, floor, steps, corners and areas around ladders or fittings. Brushing breaks up the algae layer and exposes it to the sanitizer.
Use a brush approved for your pool surface. A brush suitable for concrete or plaster may damage a fiberglass or vinyl pool.
Pay particular attention to areas with limited circulation, including behind steps and around fittings.
Shock the Pool According to the Test Results
Choose a pool-shock product that is compatible with your pool and existing water chemistry. Calculate the pool volume accurately and follow the product label for the required amount and application method.
Wear appropriate protective equipment and add chemicals exactly as directed. Never combine different pool chemicals, mix them together in a container or add water to a chemical product.
The correct treatment amount depends on the pool volume, current chlorine level, stabilizer level and severity of the algae. Avoid relying on a standard number of bags or litres without testing first.
The CDC’s pool chemical safety guidance also recommends reading all product directions, using appropriate protective equipment and keeping pool chemicals secured away from children and pets.
Keep the Water Circulating and Filter the Pool
Run the circulation system as directed during the recovery process so the treated water reaches all areas of the pool and suspended material can be filtered out.
Monitor the filter closely. Dead algae and debris can clog a filter quickly, causing the pressure to rise and water flow to decrease.
Depending on your filter type, you may need to:
- Clean a cartridge filter
- Backwash a sand or diatomaceous-earth filter
- Empty the pump and skimmer baskets
- Repeat cleaning as more algae is collected
Read our guide on how to clean and maintain a pool cartridge filter.
Vacuum and Brush Again
As the algae dies, it may settle on the pool floor or remain in areas with poor circulation. Vacuum the pool using the method recommended for your system, then brush all surfaces again.
Severe algae growth may require several rounds of brushing, filtering and vacuuming. Cleaning the pool once is rarely enough when the water has been green for an extended period.
Retest the Water
Test the water again after the treatment has circulated for the period specified on the product label.
Do not allow swimming until:
- The water is clear
- The bottom of the pool is fully visible
- The sanitizer and pH have returned to their normal operating ranges
- No active algae remains on the surfaces
- The filtration system is operating normally
If the water remains green despite appropriate treatment, have it professionally tested. Metals, an incorrect stabilizer level, filtration problems or inaccurate test results may be preventing recovery.
How to Prevent a Green Pool
Once the water is clear, consistent maintenance is the best prevention.
- Test the water regularly
- Maintain the recommended sanitizer and pH levels
- Empty skimmer and pump baskets
- Brush and vacuum the pool
- Clean the filter when required
- Maintain proper water circulation
- Test after heavy rain or periods of heavy use
- Address equipment problems promptly
If poor circulation is contributing to recurring algae, the pump, filter, sanitizer or other equipment may need inspection or replacement. Learn more about our pool equipment services or contact Earthworks to discuss an equipment upgrade.
