Iron
A staining metal, usually from fill water or corroding equipment. Judged by eye more than by band.
- Measured in
- mg/L
No range is shown because none is published for pool water. The drinking-water figure that exists is a taste-and-laundry standard and says nothing useful about a pool.
What it is
Iron dissolves invisibly in water and oxidises when it meets chlorine, at which point it comes out of solution as a rust-coloured deposit. Well water is the classic source; corroding metal in the pool's own plumbing or heater is the other.
Why it matters
Iron is not a health concern at pool levels, but it is a cosmetic one that gets worse quietly. The tell is water that turns brown or greenish-brown when you shock it — the chlorine is oxidising dissolved iron you could not see. Left alone it stains plaster, fibreglass and vinyl progressively, and staining is far easier to prevent than to remove.
How to measure it
Test when filling from a well, when stains appear, or when the water discoloured the last time you shocked it. This is not a routine weekly reading.
Test strips
- How trustworthy
- Ballpark
- Cost
- Low cost
- Effort
- Under a minute, wherever you drew the sample
A plastic strip with reagent pads on it. Dip it, hold it level, and match each pad against the chart on the bottle at the time the label tells you to. Most strips read several parameters at once.
Metal strips cover iron and are enough to confirm a suspicion before you spend anything on treatment.
Where to find it: Pool shops, hardware stores, supermarkets and online, sold both per parameter and as multi-packs. Buy strips made for your kind of water: pool strips and drinking water strips read very different scales for the same chemistry.
Free test at a pool shop
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- A trip, and a sample carried carefully
Take a sample in and they will run it, usually on a photometer, usually free, and print you a full panel. The catch is that the printout ends in a list of things to buy.
A shop metals panel gives you iron and copper together, which is what you want, since the stains they leave are told apart by colour and people guess wrong.
Where to find it: Most pool and spa retailers. Ask which parameters they cover before driving over.
Certified laboratory, mail-in kit
- How trustworthy
- Reference grade
- Cost
- Most expensive
- Effort
- Careful sampling, then days or weeks of waiting
You are posted bottles and instructions, you fill them exactly as told, and you post them back. Weeks later a report arrives with figures nobody has to interpret by eye. This is what a result you intend to act on should be built from.
Reserved for working out whether the iron is coming from the fill water or from the equipment, by testing both.
Where to find it: State-certified drinking water laboratories; your state's certification officer keeps the list, and many public health departments will point you at one.
What you can see, taste and feel
- How trustworthy
- Ballpark
- Cost
- Free
- Effort
- None
Not a measurement, and it belongs here anyway: it is free, it is continuous, and it is what tells most people to test at all. Treat it as the prompt for a number, never as the number.
Rust-brown staining below the waterline, or water that browns when shocked, is iron until proven otherwise. Blue-green in the same places is copper.
Where to find it: A clean, clear glass and a sheet of white paper.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Sample the fill water separately from the pool. The two answers lead to completely different fixes and testing only the pool cannot tell them apart.
- • Test before shocking rather than after; shocking precipitates the iron out of solution, so a sample taken afterwards reads low while the metal is settling onto the plaster.
If it is too low
Nothing to act on.
- • Nothing.
If it is too high
Staining is likely, and shocking the pool will probably discolour the water before it settles onto the surfaces.
- • Sequester before you shock. A metal sequestrant holds iron in solution so chlorine cannot precipitate it onto your surfaces, and adding shock first is what produces the brown water people then ask about.
- • Filter fill water if the source is a well; treating the pool repeatedly while refilling from the same tap is an endless job.
- • Look for the source if there is no well. A corroding heater or metal fitting will keep supplying iron regardless of what you dose.
Common questions
Why did my pool turn brown after I shocked it?
Dissolved iron. It was in the water all along, invisibly, and the chlorine oxidised it all at once. The water usually clears as it filters out, but some of that iron lands on the surfaces on its way — which is why sequestering before shocking is the order that matters.
Is iron in pool water dangerous?
No. At the levels found in pools it is a staining and appearance problem rather than a health one, which is why no aquatic health code sets a limit for it.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.