Real-life case: brown stains on the floor — metal or algae?
- The situation: brown-grey stains on the pool floor, of uncertain origin — metal, algae or organic debris?
- The direct-chlorine test settles it: a stain that fades under a chlorine tablet is organic/algal; a stain that resists is metallic.
- Metal → sequestrant and low pH (never chlorinate hard); algae → brushing + targeted chlorination.
- The right diagnosis avoids the worst: chlorinating a metal stain embeds it for good.
They appeared over a few days, on the floor, by the steps. Brown-grey stains, neither really black nor really green. Algae? Metal? A leaf that decomposed over winter? Impossible to tell by eye — and that’s the whole trap, because the right treatment depends entirely on the answer, and the wrong one can embed the stain for good. Here’s the very simple test that settles it, and what follows depending on the verdict.
Why you must absolutely not guess
Three families of floor stains, three opposite treatments. An organic stain (leaf, decomposed debris) comes off with brushing and a bit of chlorine. An embedded algae stain needs vigorous brushing and targeted chlorination. But a metallic stain (iron, manganese, copper) reacts against intuition: chlorinating it fixes it permanently, because chlorine oxidises the metal and anchors it into the surface. Charging in with chlorine “to clean it” without knowing is playing heads-or-tails with a permanent stain.
Hence the golden rule: identify before you treat.
The tablet test — 10 minutes, no special kit
The test that settles it is well known to pool people and devastatingly simple: put concentrated chlorine in direct contact with the stain (for example a few granules in a sock, rested on the stain, or a tablet held over it). You wait, you watch. A stain that fades or disappears → organic or algal, the chlorine eats it: you’re on familiar ground. A stain that doesn’t move, or even darkens → metallic: you stop the idea of chlorinating right away, and completely change your approach.
On this pool, the stain didn’t flinch. Verdict: metal — consistent with the well water used to top up in spring.
What follows, given the metal verdict
The metal treatment runs against pool instinct: no strong chlorine. You lower the pH (the metal stays more soluble), you add a sequestrant that traps the metal in a non-oxidisable form, you filter at length, and you work the embedded stains while the sequestrant acts. Caught early, this pool’s stains receded markedly in a few days; an old iron stain would have demanded far more effort, even a specific anti-metal-stain product.
For the record: had the test made the stain fade, the scenario would have been the reverse — brushing and targeted chlorination, the territory of embedded algae (see the mustard and black algae guides).
What this case teaches
A floor stain isn’t an unsolvable riddle: a 10-minute test tells you whether it’s living (algae/organic) or mineral (metal), and that answer completely reverses the treatment. The deadly trap is chlorinating a metal — a natural reflex that creates a permanent stain. The app guides the diagnosis (context: well water, post-shock, recommissioning) and rolls out the right protocol: sequestrant and low pH for metal, targeted chlorination for algae. Know before you act, always.
Frequently asked questions
How do I know if a stain on the floor is metal or algae?
The tablet test: place a little concentrated chlorine directly on the stain (a sock of granules resting on it, for example). If the stain fades or disappears, it's organic or algal. If it doesn't move or darkens, it's metallic — and you must absolutely not chlorinate further.
Do metal stains on the floor come off?
Often yes if you act early, with a suitable treatment (metal sequestrant, lowered pH) rather than with chlorine. The older and more embedded the stain, the harder it is — hence the value of a quick diagnosis.
Why must you not chlorinate a metal stain?
Chlorine oxidises the metal and fixes it: an iron or manganese stain treated with strong chlorine embeds itself lastingly in the surface. It's exactly the opposite of the reflex to have — hence the importance of identifying the nature of the stain before anything else.