Brown or rusty pool water: metals, and the chlorine trap
- Brown/rusty/tea-coloured water = dissolved metals (iron, manganese), typically after a refill (well, borehole, old mains).
- THE TRAP: a shock treatment oxidises the metals and deepens the colour — and can even stain the liner.
- The right order: sequestrant first, chlorination second.
- If the water turned brown AFTER a shock, that's your confirmation: it's metals.
Brown, rusty or tea-coloured water — especially after a refill (well, borehole, old mains) or right after a shock treatment — signals dissolved metals: iron most often, sometimes manganese. And this is the one symptom where instinct is your worst adviser: heavy chlorination makes everything worse, because chlorine oxidises the metals, deepens the colour and can permanently stain the liner. The right order is the reverse: sequestrant first, chlorination second. The amounts depend on your volume and the concentration: the CliniPool app manages this protocol, including in its “brown water” reopening plan.
Understanding the trap
Dissolved, iron is invisible: water from an iron-rich well can come out perfectly clear. But the moment an oxidiser arrives — and that’s exactly what chlorine is — the iron turns into iron oxide: rust, in suspension. The result: you treat clear water, and it turns brown. The more you chlorinate, the worse it gets, and the particles eventually settle as rust stains on the liner and joints.
That’s why the diagnosis counts double here: if your colour appeared after a shock, the culprit is confirmed with no further test needed.
The right protocol
1. Stop chlorinating. Add no more oxidiser until the metals are neutralised.
2. Test the water. pH, sanitiser, and ideally iron (specific tests). The context (recent refill, well water) weighs as much as the numbers.
3. Bring the pH towards the bottom of the range. A lower pH keeps the metals more soluble and helps the sequestrant do its work.
4. Add a metal sequestrant. This is the hero: it “imprisons” iron and manganese in a soluble, non-oxidisable form. The dose depends on the volume and the product.
5. Filter at length, then resume sanitisation gradually, watching the colour. The sequestrant doesn’t vanish in a day, but it does get used up: future refills from the same source will call for more.
6. Stains already there? Act fast: recent rust stains + a sequestrant treatment + a low pH give good results. Old ones dig in.
If it’s more like a springtime pond
Brown-green water when the winter cover comes off, with leaf deposits on the floor, is a different story: organic matter + algae + possibly metals. That’s precisely the scenario of the pool opening guide (“Also worth reading”), which the app handles with a dedicated “brown water” plan — sequester before you chlorinate, in the right order.
Frequently asked questions
Why did my water turn brown after the shock treatment?
The chlorine oxidised dissolved iron that had been invisible until then: it becomes visible (rust) and colours the water. It's the typical sign of a metal-rich refill; the answer now is to sequester, not to re-chlorinate.
Brown water = always metals?
Almost always when the colour leans rust/tea and follows a refill or a shock. Brown water with deposits on the floor after winter can also be organic matter (leaves) — the smell and the context settle it.
Can rust stains on the liner be removed?
Often yes, if you act fast, with a suitable sequestrant-based treatment and a lowered pH. The older the stain, the harder it gets.