You lift the pool cover for the first warm weekend and find murky green water instead of a swim-ready pool. A pool that has turned green after winter is often dealing with algae, but adding a random dose of chlorine before checking the water can waste time and make diagnosis harder.
Start by keeping swimmers out, clearing leaves and debris, and testing the water. Then check that the pump and filter are working, brush the pool surfaces, and treat the water according to the test results and the product label. This guide explains the steps, what to avoid, and when green water may be caused by something other than algae.
Key Takeaways
- Do not swim in green or cloudy water, especially if you cannot clearly see the pool floor.
- Remove debris, test the water and check circulation before choosing a treatment.
- Follow the pool-chemical label and your equipment instructions; do not guess at a universal shock dose.
- Retest before reopening the pool, and investigate other causes if a clear green tint remains.
Why does a pool turn green after winter?
The usual cause is algae growth after sanitiser, circulation or filtration has fallen behind. Leaves and other organic debris add to the clean-up, while water chemistry can drift when a pool is used less or equipment runs less often.
Green, cloudy water and slippery surfaces often point to algae. A pool may also look green for another reason. If the water is fairly clear, the surfaces are not slimy, or the colour appeared after adding chlorine, metals or another water-balance issue may be involved. The One Pool Care’s guide to why a pool can stay green after adding chlorine explains what to check before adding more.
Different algae can need different treatment. For example, mustard-coloured patches and dark spots may behave differently from a free-floating green bloom. See the guide to pool algae types and prevention if the colour or growth pattern does not look like ordinary green algae.
How to fix a green pool after winter
Work in order: make the pool safe, remove debris, test, restore circulation, treat, filter and retest. The correct chemical treatment depends on the pool volume, sanitizer system, current readings and the product being used, so there is no reliable one-size-fits-all dose.
1. Keep swimmers out and remove loose debris
Do not swim while the water is green or cloudy. You need clear visibility to see the floor, and algae can make steps and surfaces slippery. The Victorian Better Health Channel advises checking that pool water is clear and that the bottom is visible; a change in clarity is a reason to test and address the water before anyone swims.
If there is a cover, remove it carefully and keep leaves and dirty cover water from falling back into the pool where possible. Skim out leaves, twigs and insects with a net. Empty the skimmer and pump baskets, and check that the water level is high enough for the skimmer to draw water properly.
If the pool has heavy leaf litter or sludge, remove as much as you can with a net or vacuum before treatment. Large amounts of debris can burden the filter and consume sanitiser that would otherwise be used against algae.
2. Test the water before adding chemicals
Use a suitable test kit or arrange a professional test. At minimum, check the sanitizer reading and pH. For a stubborn or post-winter problem, also check total alkalinity and stabiliser (cyanuric acid); a professional may recommend checking for metals or other factors.
For context, Victoria’s Better Health Channel gives a pH target of 7 to 7.6 for swimming pools and notes that pH, chlorination and filtration all contribute to water quality. The suitable operating range can depend on your sanitizer, pool surface and equipment, so follow the guidance for your specific system rather than treating one number as universal.
Use the readings to decide what needs attention first. The detailed pool water testing and balancing guide explains the main measurements and how they fit together. If you are unsure how to read the results, get advice before combining corrective treatments.
3. Check that the pump and filter are moving water
A treatment cannot circulate properly if the pump is off, flow is restricted or the filter is clogged. Check that the pump is running normally, the skimmer and pump baskets are clear, return jets have flow, and valves are in the normal operating position.
Look at the filter pressure gauge and follow the equipment manual for cleaning or backwashing. Do not open a pressurised filter or work on electrical equipment while it is powered. If the pump will not prime, flow is weak, or pressure behaves abnormally, stop and address the equipment issue rather than repeatedly adding chemicals.
4. Brush the walls, floor and hard-to-reach areas
Brush the pool surfaces to dislodge algae so the treatment and filtration can reach it. Work along the waterline, walls, floor, steps, corners and shaded areas. Use a brush suited to your pool finish; a brush that is too abrasive can damage some surfaces.
After brushing, vacuum loose material from the floor. If the pool is heavily affected, check whether the vacuum should send waste to drain rather than through the filter. Follow the instructions for your pool and equipment, and keep an eye on water level if vacuuming to waste.
For a fuller cleaning sequence, use The One Pool Care’s green pool cleaning steps.
5. Treat the water using the test results and product directions
Once debris is removed and circulation is working, correct the water balance as needed and apply an algae treatment that is appropriate for your pool and sanitizer. If a chlorine shock is indicated, calculate it using the pool’s actual volume, measured water conditions and the product instructions. Do not copy a dose from a different pool or product.
Add chemicals only as directed on the label. Never mix concentrated pool chemicals together, and never combine different chlorine products. The Victorian Better Health Channel warns that pool chemicals can be dangerous when handled incorrectly, and the US Centers for Disease Control and Prevention likewise advises keeping chemicals from mixing and following safe handling directions.
Keep people and pets away while chemicals are being measured or added. Use the protective equipment specified on the label, store products in their original containers, and allow the treatment to circulate as directed. The 7-step guide to shocking a pool provides a more detailed companion walkthrough.
6. Keep filtering and clean the filter when needed
Run the circulation system for the treatment period recommended for your pool and equipment. During an algae clean-up, dead algae and fine particles can load the filter quickly, so check pressure and flow regularly. Clean or backwash the filter when its instructions or pressure reading indicate it is due.
Do not add clarifier or flocculant automatically. These products are not suitable for every filter or situation, and a flocculant may require the settled material to be vacuumed to waste. Confirm compatibility and follow the product directions before using one.
7. Vacuum settled algae and retest
As the water clears, dead algae may settle on the floor or remain suspended. Vacuum settled material slowly so it is not stirred back into the pool, then clean the filter as needed. Retest the water before making further adjustments; do not repeat a chemical dose simply because the pool has not cleared on a set timetable.
The pool is ready for swimming only when the water is clear enough to see the bottom and the sanitizer and other readings are within the appropriate range for your pool system and product guidance. Clear-looking water alone is not a substitute for a water test.
Why is my pool still green after treatment?
If the pool stays green, check the readings and equipment again before adding more chlorine. A persistent bloom can mean the sanitizer has been used up, pH or stabiliser is interfering with the treatment, the filter is not clearing particles, or circulation is poor.
If the water looks transparent rather than cloudy, consider whether metals or another cause could be tinting it. The One Pool Care’s troubleshooting article, “Added Chlorine but Pool Still Green?”, covers free chlorine, pH, stabiliser, circulation and possible metal staining in more detail.
A dark pool where the floor is not visible, recurring algae, stubborn black or mustard-coloured growth, or equipment that is not circulating water are all reasons to get a pool professional to assess the problem. Avoid draining the pool without expert advice; draining can create separate risks for some pool structures and does not correct the underlying water or equipment issue.
How to stop algae coming back next spring
A short off-season routine makes spring recovery easier. Keep leaves and debris out, maintain the sanitizer and water balance at levels suitable for your pool, and run the circulation system according to the equipment and seasonal conditions. Check the water periodically even when the pool is not being used.
After heavy rain, a large amount of leaf fall or an extended period without circulation, inspect the pool and test the water rather than waiting for a green bloom. Before closing the cover for a long period, remove debris, clean the filter and check that the pump, chlorinator and other equipment are operating as intended.
For a seasonal checklist, see The One Pool Care’s post-winter pool cleaning guide for Melbourne. It covers debris removal, algae treatment, water testing and equipment checks during a spring restart.
Frequently asked questions
Need help clearing your green pool?
If the water is heavily green, the pool floor is hidden, or the colour remains after treatment, let a pool professional test the water and check the equipment. The One Pool Care offers green-pool recovery across Melbourne, with same-day bookings when available.
