Hardly any "chlorine" is pure chlorine: they are compounds that release available chlorine as they dissolve. They differ in three things that matter a lot: how much available chlorine they contain, how they move pH and whether they contain stabiliser (cyanuric acid).
Available chlorine by product
Comparison
| Approx. available chlorine | Effect on pH | Stabiliser? | Typical use | |
|---|---|---|---|---|
| Trichlor (tablets) | Around 90 % | Lowers it | Yes | Slow maintenance, in a feeder or skimmer. |
| Dichlor (granules) | Between 55 and 62 % | Nearly neutral | Yes | Shock and quick corrections. |
| Calcium hypochlorite | Between 65 and 70 % | Raises it | No | Shock without raising cyanuric acid. |
| Sodium hypochlorite (liquid) | Usually around 10-15 % | Raises it | No | Automatic dosing and shock. |
Percentages vary by manufacturer: always check the label and adjust the dose (we explain it in what the chlorine percentage means).
When to use each
- Trichlor: convenient for everyday use in outdoor pools, because it dissolves slowly and protects chlorine from the sun. But it keeps adding cyanuric acid: watch it (recommended 30-50 ppm, legal maximum 75) and check pH, which tends to drop.
- Dichlor: dissolves fast and barely moves pH, which is why it's used for shocks and corrections. It also adds stabiliser.
- Calcium hypochlorite: a good shock option when cyanuric acid is already high. It adds calcium: in hard water, it can raise calcium hardness.
- Liquid chlorine: common in automatic dosing systems. It adds no stabiliser or calcium, but raises pH and loses strength over time and in the heat.
What about salt chlorination?
With a salt chlorinator you don't add chlorine: the cell produces it from the dissolved salt. Even so, you need to watch pH, which tends to rise, and use shock chlorine when the water gets difficult.
Frequently asked questions
Which chlorine is best for everyday use?
In private outdoor pools, trichlor tablets are the most convenient, as long as cyanuric acid is monitored. If it's already high, switch to a chlorine without stabiliser.
Which chlorine should I use for a shock?
Dichlor or calcium hypochlorite. If cyanuric acid is high, calcium hypochlorite or liquid chlorine are better, as they don't raise it.
Are "multi-action" tablets the same thing?
They are usually trichlor with other ingredients added, such as algaecide or flocculant. They contain stabiliser just like trichlor, so cyanuric acid rises too.