A pool testing 5 ppm free chlorine should not be green. When it is, one of four things is true, and they are distinguishable in about ten minutes.
1. Cyanuric acid has locked the chlorine
The most common cause by a wide margin.
CYA binds hypochlorite and protects it from sunlight. It also holds it. The DPD or FAS-DPD test measures total available free chlorine — bound and unbound together — so a pool at 100 ppm CYA and 5 ppm free chlorine has a genuine 5 ppm reading and an active fraction far too small to kill algae.
The requirement scales with CYA:
| CYA | Minimum FC to hold algae | Shock level |
|---|---|---|
| 30 ppm | 2 ppm | 12 ppm |
| 50 ppm | 4 ppm | 20 ppm |
| 80 ppm | 6 ppm | 31 ppm |
| 100 ppm | 8 ppm | 39 ppm |
| 150 ppm | 12 ppm | 59 ppm |
At 100 ppm CYA, a pool at 5 ppm free chlorine is below its minimum and losing. It reads like a healthy pool and behaves like an unchlorinated one.
Diagnosis: test CYA. If it is above 60, this is your answer. Fix: dilution. CYA does not break down — lowering it means replacing water, and holding the elevated shock level in the meantime.
How it got there: trichlor or dichlor. Every tablet adds CYA — about 6 ppm per 10 ppm of chlorine delivered — so a pool on tablets climbs into this state over two or three seasons without anyone doing anything wrong.
2. The test is lying, in the other direction
Very high free chlorine bleaches the DPD reagent. Past roughly 10 ppm on a comparator kit the color develops and then fades, and the test reads low — sometimes zero.
That is the opposite of this problem, but it produces the same confusion, and it matters because it is the case where someone adds more chlorine to a pool that already has 20 ppm.
Diagnosis: dilute the sample 1:1 with tap water, retest, double the result. Or use FAS-DPD titration, which does not bleach.
3. pH is out of range
Chlorine's killing power depends on how much of it exists as hypochlorous acid rather than hypochlorite ion, and that ratio is set by pH.
| pH | Active as HOCl |
|---|---|
| 7.0 | 75% |
| 7.5 | 50% |
| 8.0 | 22% |
| 8.5 | 9% |
A pool at pH 8.2 has roughly a third of the sanitising power of the same pool at pH 7.4. It is not enough on its own to turn a well-chlorinated pool green, but combined with slightly high CYA it closes the gap.
Diagnosis: test pH. Fix: muriatic acid, dosed to the volume. Do this before shocking — shocking at pH 8.2 wastes most of the shock.
4. It is not algae
Green water with no slime on the walls, that clears briefly when shocked and returns, is usually dissolved metal — copper, or iron.
Copper enters from an ionizer, from a copper-based algaecide, or from a heat exchanger being attacked by low-pH water. Oxidised by chlorine, it turns the water clear green and can stain plaster.
Diagnosis: brush a wall. Algae is slippery and comes away in clouds. Metal staining does not. A metals test strip confirms it. Fix: a chelating or sequestering agent, and then finding the source — most often a pool that has been running acidic and eating its own heater.
The order to work through it
- Test CYA. Above 60 with a green pool, that is the answer.
- Test pH. Correct it to 7.2–7.4 before anything else.
- Brush a wall. Slime is algae. No slime is metal.
- Dilute-and-retest the chlorine if the reading looks implausible either way.
Then, and only then, shock — to the level the CYA table demands and held there until the chlorine stops disappearing overnight, which is the actual endpoint. How much shock that takes depends on the same CYA number, which is why testing it comes first.
Why "more algaecide" is the wrong reflex
Algaecide is a preventative and a supplement. It does not fix a pool whose sanitiser is chemically unavailable, and buying it is how a chemical budget doubles while the actual cause — one number on a test kit — goes unread for the whole season.