Your pool can look perfectly fine one week, then suddenly turn cloudy, chew through chemicals, or leave stubborn scale marks around the tiles the next. You adjust the pH, add the usual treatments, and somehow the water never quite seems to settle.
In many cases, the issue starts with your pool’s alkalinity.
It's one of the most overlooked parts of pool chemistry, quietly affecting everything from pH stability to chlorine performance and day-to-day pool maintenance. The good news is that once you understand what it does and how to keep it in range, maintaining clear, balanced water becomes far more straightforward.
Key takeaways
-
Pool alkalinity measures your water's ability to resist sudden pH changes
-
The ideal range for most pools is 80 to 120 ppm, though fibreglass and vinyl pools sit higher
-
Low alkalinity causes pH bounce, corrosion, and eye irritation
-
High alkalinity leads to cloudy water, scale build-up, and reduced chlorine effectiveness
-
Sodium bicarbonate raises alkalinity; dry acid or muriatic acid lowers it
-
Testing at least weekly, and after rain or heavy use, keeps things manageable
What is Total Alkalinity?
Total alkalinity measures the concentration of dissolved alkaline substances in your pool water, mainly bicarbonate and carbonate salts. In practical terms, it acts as a buffer that helps prevent sudden swings in your pool’s chemical balance.
When alkalinity sits within the correct range, maintaining balanced conditions becomes much easier and less prone to rapid fluctuations. Alkalinity is measured in parts per million (ppm), which indicates the concentration of dissolved material in the water.
Why Pool Alkalinity Matters
Pool alkalinity and pH work closely together, but they measure different things.
pH measures how acidic or alkaline the water is at a specific moment, ideally between 7.2 and 7.6. Alkalinity helps stop pH levels from fluctuating too rapidly due to rain, chemical additions, heavy pool use, or environmental changes.
When alkalinity falls out of range, pH often becomes unpredictable, making the pool harder to balance and sanitise effectively.
The Link Between Alkalinity and pH Stability
You can test your pool and get a perfectly normal pH reading of 7.4, but if alkalinity is too low, that balance often won’t last.
This is what many pool owners refer to as ‘pH bounce’, where pH levels fluctuate unpredictably even with regular maintenance.
Why Alkalinity Should Be Addressed First
Correcting alkalinity first creates a more stable foundation for the rest of your pool chemistry. It makes pH easier to manage, improves sanitiser performance, and helps prevent recurring water balance issues.
What are the Ideal Pool Alkalinity Levels?
For most backyard pools, the recommended range depends on the surface type:
-
Concrete, plaster, or tiled pools: 80 to 120 ppm
-
Fibreglass and vinyl pools: 125 to 150 ppm, to provide extra buffering for surfaces that are more sensitive to chemical variation
-
Spas: 125 to 150 ppm regardless of surface type, given the higher temperatures and heavier bathing loads involved
Perth's mains water and bore water tend to carry more minerals than water in some other parts of Australia. If you're topping up regularly, particularly through summer, your levels can gradually rise without any obvious trigger.
Signs Your Pool Alkalinity is Too Low
When alkalinity drops below 80 ppm, the water becomes unstable. pH starts to fluctuate, sometimes daily. The pool may look fine, but chlorine isn't sanitising as effectively as it should be during those swings.
Over time, acidic water starts working on the pool itself. Metal fittings corrode. Plaster and grout begin to show wear. Anyone swimming will notice it too, as eye irritation and skin discomfort are common early signs.
The most common causes include:
-
Heavy rainfall
-
Overuse of dry acid or low-pH chlorine tablets
-
Prolonged high bather loads
Perth summers often combine all three factors at once. Long periods of heat, heavy pool use, and regular chemical adjustments can slowly push alkalinity out of range before most pool owners notice.
Signs Your Pool Alkalinity is Too High
High alkalinity can make the water resistant to adjustment, meaning pH becomes harder to bring down and easier to lose control of, which makes chlorine less effective.
Common signs of high alkalinity include:
-
Cloudy or milky water that doesn't respond to shock treatments
-
White scale forming on tiles, along the waterline, or inside pipes
-
Reduced flow from scale build-up on heating equipment
-
pH that climbs back up shortly after being corrected
How to Test Pool Alkalinity
Regular testing is the easiest way to spot small problems early before they turn into bigger problems.
Test Strips
Test strips are a simple option for routine weekly checks. They’re quick, convenient, and useful for monitoring alkalinity, pH, and chlorine levels at home.
Liquid Test Kits
Liquid test kits offer more accurate readings and are better suited to complete water testing. Most kits measure alkalinity, pH, and chlorine together, making them ideal for regular pool maintenance.
Professional Water Testing
Professional water testing provides a more detailed analysis, including calcium hardness, stabiliser, salt, and total dissolved solids; all factors that influence how stable and manageable your pool water remains.
How to Raise Pool Alkalinity
If alkalinity is below the recommended range, sodium bicarbonate is the standard treatment. It’s the same active ingredient found in most pool alkalinity increasers. It raises total alkalinity with minimal impact on pH, which makes it the easier option to work with.
-
Test the water and confirm your current alkalinity reading
-
Calculate the required amount based on your pool's volume; your pool specialist can help with this if you're unsure
-
Add no more than half to three-quarters of the estimated amount at a time, distributed evenly across the pool surface
-
Run the pool pump for at least two hours to circulate the chemicals
-
Wait 6-24 hours, then retest before adding more
Don't add other adjustment chemicals until the alkalinity has stabilised.
How to Lower Pool Alkalinity
Dry acid (sodium bisulfate) and muriatic acid are both effective for bringing high alkalinity down. Both lower alkalinity and pH at the same time, so slow and steady is the right approach.
-
Make sure the pump is running before adding anything
-
Add the acid in measured doses, roughly one litre per 55,000 litres of pool water for every 10 ppm reduction needed
-
Test pH during the process. If it drops below 7.2, pause and allow it to stabilise before continuing
-
Retest alkalinity before deciding whether to add more
Pool acids need careful handling. Always add acid to water, not the other way around, and wear gloves and eye protection.
How Often Should You Check Your Alkaline Levels?
Like most aspects of pool chemistry, consistency matters more than large corrections. Regular testing helps you spot small shifts early, before they turn into cloudy water, unstable pH, or ongoing pool maintenance issues.
Once a week is the minimum. During summer, or any time the pool gets heavy use, testing more frequently is worth it.
After significant rain, test before the next swim. The same applies after topping up with fresh water, which can shift alkalinity in either direction depending on your water source.
A monthly professional water test is a good complement to your own weekly checks, particularly for picking up slower-moving changes that interact with alkalinity gradually over time.
Keep Your Pool Balanced and Problem-Free
Keeping alkalinity balanced is one of the simplest ways to maintain clearer water, more stable chemistry, and a healthier pool overall. When levels move too high or too low, it can lead to unstable pH, reduced chlorine effectiveness, scaling, cloudy water, and ongoing maintenance issues.
With regular testing and small adjustments when needed, most alkalinity issues are straightforward to manage before they develop into larger water balance problems. If your water chemistry keeps drifting out of balance, professional pool maintenance can help identify the underlying cause and get your pool back into balance faster.
Get in touch with Pool Assist for expert advice, water testing, and practical support tailored to your pool system.
Frequently asked questions
Q. What's the difference between pool alkalinity and pH?
A. pH measures how acidic or alkaline your water is right now. Alkalinity measures how stable that pH level is. They work closely together, but alkalinity should always be checked and corrected before adjusting pH.
Q. Can I use baking soda from the supermarket to raise pool alkalinity?
A. Yes. Baking soda is sodium bicarbonate, the same active ingredient used in most alkalinity increasers. Pool-grade products are simply packaged and measured specifically for pool use.
Q. Why does my pool pH keep changing even after I adjust it?
A. In most cases, this points to low alkalinity. When alkalinity is out of range, pH becomes unstable and can continue shifting even after treatment.
Q. How does Perth's water affect pool alkalinity?
A. Perth tap and bore water often contains higher mineral levels, which can gradually increase alkalinity over time, particularly during summer when pools require more topping up.
Q. When should I call a professional instead of adjusting it myself?
A. If alkalinity keeps drifting out of range, your water stays cloudy, or balancing chemicals aren’t solving the issue, a professional water analysis can help identify underlying problems that standard home testing may miss.