Where the metals come from
Metals arrive with fresh make-up water — every top-up, every refill, every hose that goes in to replace evaporation. They are dissolved and invisible. Whether the source is a municipal supply or a private well, iron, copper, manganese, cobalt and silver can all be present in trace amounts, and the EPA's basic information on drinking water is a useful orientation to what a supply may contain and why it varies so much by region.
Other routes matter too:
- Corroding equipment. Heater exchangers, copper plumbing, metal fittings and ladders shed metal into aggressive water. This is why low pH is not merely uncomfortable — it is actively dissolving the pool's own hardware into the water.
- Copper-based algicides. Effective, and they are also a deliberate addition of the metal most likely to stain. Used repeatedly without monitoring, they accumulate.
- Mineral sanitiser cartridges, which release copper or silver by design.
- Well water, which is frequently far higher in iron and manganese than any municipal supply.
Over time the metals accumulate, and the more that accumulates the greater the chance that they will eventually come out of solution. Once out of solution they either cloud the water or, worse, plate out onto the pool's surfaces and stain.
The two things that bring metals out of solution
1. Shocking
When a pool is shocked — with chlorine or with a non-chlorine oxidiser — the shock oxidises everything in the water, and that includes dissolved metals. Oxidised iron becomes rust; oxidised copper becomes the blue-green of verdigris. This is why staining so often appears the morning after a shock treatment, and why the owner reasonably but wrongly concludes the shock product caused it. The metals were already there. The shock simply revealed them.
2. Aggressive water
Metals also come out of solution, or are left exposed, when pH, total alkalinity and calcium hardness are allowed to drift low — pH below about 7.2, total alkalinity below about 80 ppm, calcium hardness below about 100 ppm. The water becomes aggressive, and metals become far more susceptible to oxidation.
The trap here is that owners frequently do not test at all and instead rely on how the water looks. The water is clear, so everything must be fine. Aggressive water is perfectly clear right up until the moment it is not.
This is the mechanism behind the most familiar version of the problem: blonde hair turning green. Low pH helps bring copper out of solution, and the copper plates out onto hair. It is not "too much chlorine", which is the usual explanation given.
Identifying the metal by colour
| Colour | Usual metal | Common source |
|---|---|---|
| Rust brown, orange-brown | Iron | Well water, corroding steel, old plumbing |
| Blue-green, teal, sometimes black | Copper | Copper plumbing, heater exchanger, copper algicide, mineral cartridge |
| Purple or dark crystalline deposits | Copper combined with cyanurate | Copper plus very high cyanuric acid |
| Purple-black, grey-black | Manganese | Well water |
| Grey-black, sometimes with a metallic sheen | Silver | Mineral sanitiser systems |
| Green-brown, slimy, comes off when brushed | Not a metal — organic | See algae |
The simplest field test distinguishes metal from organic: brush the mark firmly. Organic growth lifts. A metal stain is in the surface and will not move. A second test is to hold a chlorine tablet against a small area of the stain for a minute — organic marks usually lighten, metal stains generally do not, and some darken.
Prevention
Prevention is straightforward and it is almost entirely about testing.
- Test your fill water before it goes in, particularly from a well, and particularly before a large top-up or a refill.
- Use a sequestering agent when adding significant fresh water. A sequestrant binds dissolved metals and holds them in solution so they cannot oxidise out. It is a preventive measure, not a cure, and it needs periodic re-dosing because it is gradually consumed.
- Keep the balance in range, especially calcium hardness and pH, so the water is neither aggressive toward your own equipment nor primed to precipitate.
- Never let pH sit low. It is the fastest way to dissolve a heater into your pool.
- Add sequestrant before shocking if you know metals are present, and shock in the evening so oxidation happens without simultaneous sunlight.
- Watch copper algicide use. Repeated dosing without monitoring accumulates copper indefinitely.
- Avoid very high cyanuric acid if copper is present; the combination is what produces purple crystalline deposits.
Removing existing stains
- Identify it first. Ascorbic acid treatments lift iron and copper; they do nothing for organic marks or for scale. Applying the wrong treatment wastes a great deal of product.
- Lower the sanitiser level before a stain treatment. Stain removers are reducing agents and free chlorine will simply consume them.
- Treat. Follow the product's directions for the true pool volume — the volume tables matter here more than almost anywhere else, because these treatments are expensive.
- Sequester immediately afterwards. This is the step people miss. A stain treatment returns the metal to solution; without a sequestrant to hold it there, it will simply plate back out within days and the whole expense is repeated.
- Rebalance, then slowly restore the sanitiser level over a few days rather than shocking straight back to normal.
Handling note. Stain treatments and acids are concentrated chemicals. Never mix products, never add water to acid, wear eye protection, and work in ventilated conditions — the general principles are set out in the OSHA overview of chemical hazards.
When the stain is not a metal
Two look-alikes are worth ruling out. Scale is a mineral deposit — usually calcium — sitting on top of the surface rather than in it; it feels rough, sometimes sharp, and is caused by high pH and high hardness rather than by metals. Organic staining from leaves, berries, acorns and worms sits in the surface like a metal stain but is usually leaf-shaped or spot-shaped, is brown rather than orange, and responds to oxidation rather than to reduction.
If a treatment for one has no effect at all, that is diagnostic information rather than a reason to double the dose.