How to fix a green pool, step by step | Perth guide

Your pool has gone green because algae has taken hold in the water. Algae blooms when free chlorine drops below the level needed to control it, and warm water speeds that up, so a clear pool can turn green in a couple of days. In Perth that usually happens between September and October, when the water starts warming and pump hours are still on winter settings.

Fixing it is a sequence. Each step below depends on the one before it, so work through them in order.

1. Why your pool turned green (spoiler: it isn’t dirt)

If you are asking yourself why is my pool green, the short answer is algae growth driven by low chlorine levels. Chlorine is consumed by sunlight, heat, rain and swimmers, and once free chlorine falls low enough, algae multiplies faster than the remaining chlorine can kill it. That is why skimming and vacuuming on their own will not clear a green pool or solve how to get rid of algae in pool water for good. You have to correct the chemistry first and then let the filter remove what the chemistry kills. Shaded corners, steps and the area behind a ladder usually go first, because circulation is weakest there.

2. Test before you treat (guessing costs more)

Test the water before you add anything. The readings decide what goes in and how much, and guessing almost always costs more than testing does. A strip or a home kit will give you pH, total alkalinity and free chlorine, which is enough to start. If your kit has been sitting in a hot shed since autumn, treat the results with caution, because reagents and strips degrade. If you would rather have the numbers confirmed, bring a water sample into our Claremont store and we will test it for free.

3. Balance pH first, then shock

Correct pH before you add chlorine, because chlorine does very little work outside the right range. Aim for pH between 7.2 and 7.6, and total alkalinity between 80 and 120 ppm. Alkalinity buffers pH, so set alkalinity first and pH will hold once you adjust it. With the water balanced, you can shock the pool to effectively remove algae from pool surfaces and water. How much you need depends on your pool volume and current chlorine reading, so there is no single figure that works for every pool. Bring your readings into Claremont or give us a call, and we will help you calculate the exact dosage so you do not waste money guessing.

4. Run your filter longer than you think you need to

Run the pump continuously while learning how to fix a green pool, not on your usual timer. Dead algae has to be filtered out, and a pool running six hours a day will not turn over enough water to do it. Keep it running 24 hours a day until the water clears, then return to your normal schedule. Check the filter before you start, because a unit that is already loaded will slow the whole job down. If your sand, cartridge or media filter is struggling to keep up, bring it by the shop and our team will check if it needs a deep clean or media change.

5. Brush and vacuum to waste (how to clean a green pool properly)

Knowing how to clean a green pool properly means brushing before you vacuum. Algae clings to surfaces, and brushing lifts it into suspension so the chlorine can reach it and the filter can catch it. Then vacuum to waste rather than back through the filter, so heavy dead algae leaves the system instead of clogging it. Keep an eye on your water level while you do this and top up as it drops. A pool cleaner will handle the routine work once the water is clear, but the first pass after a bloom is usually a manual job.

6. Backwash, retest, repeat if needed (no overnight miracles)

Backwash when the filter pressure climbs above its clean starting reading, then retest and go again. A pool that has gone properly green rarely clears in a single cycle. Despite what online quick-fix guides promise about how to clear a green pool overnight, you should not expect a 12-hour turnaround. Two to five days is realistic, depending on how far the bloom has gone and how well the filter keeps up. Retest pH and chlorine each day and hold the balance while the filter does the work.

7. When to bring a water sample into Claremont

Bring a sample in if the water is still green after several full cycles, or if your chlorine will not hold. That pattern usually points to something a strip does not show, such as high stabiliser, phosphates feeding the algae, a chlorinator cell at the end of its life, or a filter that is undersized for the pool. Take the sample from about elbow depth, away from the return jets, and bring it in soon after you collect it. Nicole and the team at our Claremont store will test it and tell you what the water actually needs.

Getting ahead of it next spring 

Most green pools in Perth are a winter problem that only shows up in spring. If you would rather not run this sequence again next year, work through our spring pool start-up checklist before the water warms up.