Pool Water Chemistry Explained: pH, Chlorine and Alkalinity for Auckland Pool Owners
The core numbers behind a healthy pool, and why testing beats guessing every time.
Cooper's background before pool care was aircraft engineering and the fire service, and one habit carried straight over: you don't declare something fine until you've actually measured it, not just eyeballed it. Pool chemistry rewards that approach more than most people realise. A pool can look completely clear and still be out of balance, and a few numbers, tracked properly, explain almost everything that goes wrong with a pool. Here's what those numbers actually mean.
How Often Should You Actually Test?
At least once a week for a home pool, more often in summer or after heavy use, rain, or a pool party. A professional visit typically includes a full test every time, with chlorine, pH, and alkalinity as the core three. Skipping tests doesn't save time, since problems caught early are quick fixes and problems left for weeks usually aren't.
We've seen what happens when that testing lapses. One pool that hadn't been properly looked after had its salt chlorinator left running at 100%, which pushed chlorine levels very high. Bringing the output back down and getting the water back into balance only happened because someone actually tested it, rather than assuming a clear-looking pool was a fine one.
Getting pH Right Matters More Than People Think
The target range is 7.2 to 7.6, with 7.4 to 7.6 the sweet spot. Below that range irritates skin and eyes and corrodes equipment over time. Above it, chlorine stops working properly even if the chlorine reading itself looks fine on the test strip. pH is the setting that makes every other chemical actually do its job, which is why it's checked first, not last.
We've also come across pools where the automatic acid dosing wasn't set right, which dragged pH too low without anyone noticing until it was tested. Correcting the dosing settings and bringing pH back into range is a straightforward fix, but only once it's actually caught, which again comes back to testing rather than assuming the system is looking after itself.
Free Chlorine vs Total Chlorine, What's the Difference?
Free chlorine is the active sanitiser still available to kill bacteria and algae. Total chlorine is free chlorine plus combined chlorine, which has already reacted with contaminants in the water and isn't doing any more sanitising work. A gap between the two readings usually means it's time to shock the pool, since that combined chlorine needs to be broken down rather than left to build up.
That Strong Chlorine Smell? It Means the Pool Needs More Chlorine, Not Less
It's one of the most common misreadings in pool care. A strong chemical smell usually means too little free chlorine, not too much. The smell comes from combined chlorine, chloramines, that's already reacted with contaminants in the water, not from clean water having "too many chemicals" in it. A properly balanced pool with plenty of free chlorine available barely smells like anything at all. If a pool smells strongly of chlorine, treat it as a sign the water is dirtier than it looks, not a reason to add less next time.
Why Total Alkalinity Is the Setting Behind the Setting
Total alkalinity is the water's ability to resist pH swings, and it's easy to overlook because it doesn't show up in the water the way algae or cloudiness does. Keep it in the roughly 80 to 120 ppm range and pH stays stable and predictable. Let it drift outside that range and pH will bounce around no matter how carefully it's corrected, which is why a pool that seems to need constant pH adjustment often actually has an alkalinity problem underneath it.
What Causes Scale Buildup Around the Pool
Scale, the white, crusty deposits that show up along tile lines and on equipment, comes from high calcium hardness combined with high pH or alkalinity. It's a slow-building problem rather than a sudden one, but left untreated it can damage surfaces and clog fittings, so it's worth catching on a test rather than waiting until it's visible.
Can Balanced Water Still Irritate Skin and Eyes?
Yes, and it's usually pH, not chlorine, that's actually responsible. Irritation gets blamed on chlorine by default, but a pool can have a completely normal chlorine reading and still sting eyes or dry out skin if pH is out of range. It's another reason pH gets checked alongside chlorine rather than assumed to be fine.
Clear Doesn't Always Mean Safe
Clarity alone doesn't confirm a pool is actually safe to swim in. A pool can look completely clear while carrying unsafe bacteria levels if chlorine has dropped too low without anyone noticing, since bacteria growth doesn't always show up visually the way algae does. Regular testing, not a glance at the water, is the only reliable way to know where a pool actually stands.