What pool chemicals actually do

When you add chlorine to water it splits into two forms. One is hypochlorous acid. The other is the hypochlorite ion.

Both show up on your test as free chlorine. Only one of them does much work. Hypochlorous acid is roughly a hundred times better at killing things than the hypochlorite ion.

So two pools can test at exactly the same chlorine level and behave completely differently. One stays clear. The other keeps going cloudy, and the owner keeps adding chlorine that was never going to help.

What splits the chlorine between those two forms is pH.

At around pH 7.5 it is roughly an even split — about half your chlorine in the useful form, half in the lazy one. Push the pH up and the balance tips the wrong way. Bring it down and more of the chlorine becomes the form that actually kills things.

This is the single most useful thing to understand about pool water. pH is not a comfort setting. It is the dial that decides whether the chlorine you paid for is doing anything.

It is also why we test pH before we touch chlorine, every visit. Adding chlorine to a pool with drifted pH is pouring money in the skimmer.

If pH is the dial, total alkalinity is what stops the dial spinning.

Alkalinity is a buffer. It soaks up the things that would otherwise shove your pH around — rain, topping up, swimmers, the chlorine itself.

When alkalinity is too low, pH bounces. You correct it on Monday and it has moved by Thursday. People blame the pH and keep dosing it, when the problem is underneath.

When it is too high, the opposite: pH gets stubborn and drifts upward, and correcting it takes far more acid than it should.

Commonly quoted targets sit somewhere around 80 to 125 parts per million for a pool, though the right number depends on your surface and your water.

Calcium hardness is the one people ignore, because it does not affect how the water looks this week.

Water that is short of calcium will go and find some. It takes it out of whatever it is sitting in — the grout, the render, a pebble interior, the tile line. That is not a water problem you fix later. That is your pool surface dissolving slowly.

Too much calcium goes the other way and starts depositing. You see it as scale at the waterline, and inside the salt cell, where it shortens the life of an expensive part.

Recommended ranges vary by surface type, and a concrete or pebble pool wants more than a fibreglass one.

Stabiliser, or cyanuric acid, wraps around chlorine and shields it from ultraviolet light. Without it, a Sydney summer day will strip most of your chlorine out of the water in hours.

So it is genuinely necessary here. The catch is that it does not leave. Chlorine gets used up. Stabiliser accumulates, season after season, and the only real way to bring it down is to dilute the water.

Once it climbs high enough it stops protecting the chlorine and starts holding it back. The pool tests as having chlorine and still goes green.

That is common enough that we gave it its own page: why your chlorine will not hold.

A salt pool is not an alternative to chlorine. The cell takes the salt in the water and makes chlorine out of it, continuously, while the pump runs.

Everything above still applies. pH still decides how much of that chlorine is useful. Stabiliser still builds up. Calcium still scales, and in a salt pool it scales inside the cell.

What changes is that the chlorine arrives steadily instead of in doses, and that you now own a consumable part that wears out. More on that in salt pools: what people get wrong.

Balance first, sanitise second. Always that way round.

So: alkalinity, then pH, then chlorine. Correcting chlorine before the water underneath it is right is the most common and most expensive mistake in pool care.

Calcium and stabiliser get checked less often, because they move slowly. But they are the two that cost real money when they are ignored for years.

If you would rather not track all of it, that is what water testing is for. We write the numbers down each visit so you can see what moved.