Why a pool can hold chlorine and algae at the same time

A pool reads 2 ppm of free chlorine and is visibly green. The owner tests again, gets the same answer, and concludes the test kit is broken.

The kit is fine. Two parts per million is plenty of chlorine in one pool and nowhere near enough in another, and the difference is a chemical most owners have never tested for.

Stabiliser protects chlorine and slows it down

Cyanuric acid, sold as stabiliser or conditioner, binds loosely to chlorine in the water. That bond shields it from ultraviolet light, which otherwise destroys a large share of free chlorine within hours in direct sun. Outdoors it is close to essential.

The catch is that bound chlorine is not available to do the work. It is a reserve rather than an active sanitiser, and the share held in reserve rises as the stabiliser level rises. So the same 2 ppm reading describes a pool with plenty of active chlorine at 20 ppm of stabiliser and a pool with almost none at 100.

The ratio that actually matters

The working guideline that has emerged from this is to hold free chlorine at roughly 7.5% of the cyanuric acid level as a minimum.

At 30 ppm stabiliser that is about 2.25 ppm chlorine. At 50 it is around 3.75. At 100 it is 7.5 ppm, which is a level most owners would consider alarming and which is simply the maintenance figure for that pool.

This single relationship explains most green pools that test positive for chlorine. The reading was never wrong. It was being compared against a fixed target when the real target had moved.

Stabiliser creeps up on its own

Nobody adds cyanuric acid deliberately more than once. It arrives inside the chlorine.

Trichlor tablets and dichlor granules are both stabilised products, so every dose adds a little more cyanuric acid to the water. The chlorine gets consumed and the stabiliser does not, so a pool run all season on tablets climbs steadily, and by late summer it can be well past 100 ppm with the owner never having bought a bag of conditioner.

The only practical way back down in a home pool is to drain some water and refill. That is the whole argument for testing it a few times a season rather than once at opening.

What to do about it

Test for cyanuric acid, then set the chlorine target from it rather than from a number on a chart.

If the stabiliser is in a sensible range, somewhere around 30 to 50 ppm for most outdoor pools, hold chlorine around 7.5% of it as a floor and rather higher as a working level. The pool chemical calculator turns a target rise into an actual dose for the product you have, in ounces or grams, which is the step where most dosing goes wrong because the strengths differ so much between products.

If the stabiliser has climbed too high, the fix is dilution. Drain and refill a portion, retest, repeat if needed, and switch the routine over to an unstabilised chlorine for the rest of the season so it stops climbing.

Switching products is the long term answer

Liquid chlorine and cal-hypo carry no cyanuric acid, so they raise chlorine without adding to the problem. Liquid is the usual choice for regular dosing in a pool that already has enough stabiliser.

The trade is convenience. Tablets sit in a floater and do their job unattended for days, which is exactly why they are so popular and exactly how pools end up over-stabilised. Using tablets while away and liquid the rest of the time is a reasonable compromise that keeps the creep slow.

The test kit matters more than the chemicals

Strip tests are convenient and approximate, and in this particular relationship approximate is not much use. A stabiliser reading that could be 40 or could be 90 does not let you set a chlorine target at all.

A drop-based test kit that reads free chlorine and cyanuric acid separately costs more once and removes the guesswork permanently. For a pool that keeps going green despite apparently adequate chlorine, it is usually the cheapest thing that will actually fix it.