Two tablets ride in every Marlin service cart, and neither one is for the water. A trichlor chlorine tablet and a crushed vitamin C tablet are the fastest honest answer to a question we hear all summer: what is that mark on the bottom of my pool? Hold one, then the other, against the stain for a minute each, and the stain itself will usually tell you what it is — which matters, because the treatment that erases one kind of stain can set another kind permanently.
Why the wrong guess costs you
Stain treatment is one of the few pool jobs where doing something can be worse than doing nothing. Shock a metal stain and the extra chlorine oxidizes the dissolved iron or copper, darkening the mark and bonding it more tightly to the plaster. Pour ascorbic acid at an organic stain and you’ve spent money, crashed your chlorine, and changed nothing. We regularly see pools that have been through two or three wrong treatments before anyone asked the first useful question: which of the three pool stain types is this?
The three families, at a glance
Nearly every stain we encounter in Scottsdale-area pools belongs to one of three families, and each of the three pool stain types has a tell:
| What you see | Where it shows up | Likely family | Quick check |
|---|---|---|---|
| Green-brown patch, irregular edges | Under trees, where debris settles, shaded corners | Organic (leaves, algae remnants) | Chlorine tablet lightens it |
| Rust-brown, tea-colored, or grey-black shadowing | Broad areas, around fittings, below returns | Metal (iron, manganese) | Vitamin C tablet lightens it |
| Blue-green streaks | Below returns, near heater plumbing | Metal (copper) | Vitamin C test; check heater and algaecide history |
| White-grey crust or rough film | Waterline tile, spillways, swim-outs | Mineral (calcium scale) | Feels rough; doesn’t respond to either tablet |
The tablet tests are worth doing even when the color seems obvious. Brown, in particular, is an unreliable witness — it’s the shared uniform of old leaves and dissolved iron, and the two demand opposite treatments.
A Rio Verde iron stain that looked like algae
Earlier this summer we took over a pool out in Rio Verde where the owner had been fighting “algae” on the steps and shallow end for a month — brushing, shocking, repeat. The vitamin C tablet cleared a bright spot in about forty seconds: iron, not algae. Out there the story usually starts with the fill water. A lot of properties out there run on hauled water or private wells, and that water can carry dissolved iron and manganese that city-supplied Scottsdale pools rarely see. Every top-off had been quietly raising the metal level, and every shock had been developing it like film.
The fix ran in the opposite direction from everything the owner had tried: chlorine down, a measured ascorbic acid treatment to lift the stain, then a sequestrant to hold the dissolved metal in solution, and a sequestrant maintenance dose folded into the weekly service schedule in Rio Verde going forward. The steps came back over two visits.
Matching treatment to family
Organic stains are the forgiving ones. Raise chlorine, brush hard, give it a few days. For a concentrated spot, a trichlor tablet held on the stain (briefly — it can etch if parked too long) or granular chlorine brushed over it accelerates things. If the same corner keeps staining, the real fix is upstream: trim the tree, empty the leaf trap more often, adjust circulation so debris doesn’t settle there.
Metal stains reverse the logic. Drop chlorine to the low end of range, apply ascorbic acid, and watch the stain fade — often within hours. The part most owners skip is the follow-through: the metal is still in the water, just invisible again. Without a sequestrant, and without addressing where the metal came from (fill water, a corroding heat exchanger, copper-based algaecide), the stain reappears on schedule with the next shock.
Mineral scale is less a stain than a deposit — our hard Valley water leaving calcium behind as it evaporates. Light scale responds to careful acid-side water balance and elbow grease; the heavy waterline banding you see on tile is its own project, closer to tile and surface cleaning than to chemistry, and we’ve covered it separately.
Red flag — draining and acid washing: when a stain won’t budge, the internet’s next suggestion is usually “drain it and acid wash.” Treat that as a licensed-pro decision, not a weekend project. An acid wash strips a layer of plaster along with the stain, the runoff has to be neutralized and disposed of legally, and draining an in-ground shell in desert soil at the wrong moment can crack or float the pool. Nearly every stain in this article is treatable with the water right where it is.
Keeping them from coming back
Prevention looks much the same across all three pool stain types — a short list of unglamorous habits:
- Net debris before it sinks, and don’t let leaf piles winter on the floor — tannin stains are rented by the hour and owned by the week.
- Test fill water if you’re on a well or hauled supply; if it carries metals, run sequestrant as maintenance, not as a rescue.
- Keep chlorine and pH in range week over week — most organic staining starts during a sanitizer lapse, and scale accelerates when pH drifts high.
- Brush weekly, even where the pool looks clean. Stains that get brushed young rarely become projects.
That week-over-week consistency is, frankly, the quiet argument for having a pro on the pool — spotting a shadow on visit three beats treating a stain on month three. If you’d rather hand the whole rotation off, here’s what that covers, or call us at 480-404-5554 with a photo of whatever your pool is doing.
Industry testing and treatment standards for pool surfaces and water quality are maintained by the Pool & Hot Tub Alliance, whose guidance informs how we approach surface care.
Stain questions we hear
What causes brown stains on pool steps?
Brown on steps is a coin flip between organic staining — leaves or debris that sat on the plaster — and iron, which often shows up after fill water introduces metals. Location helps: organic stains follow where debris settles; iron tends to shadow broader areas and darken after shocks. A crushed vitamin C tablet held against the spot settles it — iron lightens within a minute, organics don’t.
Will shock remove pool stains?
Only organic ones. A strong dose of chlorine plus brushing breaks down leaf tannins, algae remnants, and similar staining, often within days. Metal stains respond in the opposite direction — extra chlorine oxidizes dissolved metals and can make the mark darker and more fixed. That’s why identifying the stain family before treating matters more than the treatment itself.
What is the vitamin C test for pool stains?
Hold a crushed ascorbic acid (vitamin C) tablet against the stain for about sixty seconds. If the spot visibly lightens, you’re looking at metal — most often iron. Its counterpart is the trichlor test: a chlorine tablet held on the stain lightens organics. Between the two, you can sort the large majority of stains without draining anything.
Are pool stains permanent?
Most respond well when matched to the right treatment, especially caught early — and that holds across all three pool stain types. Organic stains usually lift completely. Metal stains fade substantially with ascorbic acid, though older ones can leave a shadow. Heavy mineral scale takes professional equipment. Stains etched into aging plaster are the stubborn minority — that’s a resurfacing conversation, not a chemical one.
Do stains mean I need to drain and acid wash the pool?
Rarely, and it should be the last resort — an acid wash removes a thin layer of plaster along with the stain, and draining an in-ground pool in desert soil carries real structural risk if done at the wrong time. Chemical treatment in a full pool resolves most staining. Save the drain for pools that need it for other reasons.
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This article is for general educational purposes. Pool chemistry, equipment, and electrical work involve real safety risk — if anything in your system is unfamiliar, call a licensed pool professional before making changes. Marlin Pools is licensed in Arizona, ROC #331368.
