Diagnose the stain, then buy. Not the other way round.

The test that saves you buying the wrong thing

Metal and organic staining both show up as discolored patches on a pale vinyl liner and they respond to completely opposite chemistry. Before you buy anything, find out which one you have.

1. 1

   Hold a chlorine tablet against the stain

   Thirty seconds is enough, with the pump off. Use a gloved hand or press it against the spot with a pole. You are looking for a change in the vinyl, not in the water.
2. 2

   If the stain lightens, it is organic

   Leaves, algae shadow, worms, berries — anything that was alive. Chlorine breaks it down, so a concentrated dose against it does visibly something in half a minute.
3. 3

   If nothing happens, it is metal

   Iron, copper or manganese that has oxidized out of solution and deposited on the liner. Chlorine does nothing to it, and more chlorine makes it worse rather than better.
4. 4

   Check the color as a cross-check

   Rust brown and reddish usually means iron. Blue-green or teal usually means copper. Purple-black usually means manganese. Our reading water color guide covers the full set.
5. 5

   Then buy exactly one product

   Not a shelf of them. The two categories are mutually exclusive, and the most common way people spend money on this problem is by treating both.

Metal stains: what to buy and why

The products that work on vinyl are citric-acid based, commonly sold as a powder in one- or two-pound tubs, and rated for vinyl liners, fiberglass and metal fittings. The reason the category is narrow is that vinyl is the constraint: stronger acid cleaners exist and will take the stain off along with some of the liner.

Three things to know about using one.

It drops pH while it works. That is not a side effect, it is the mechanism — the acid is what puts the metal back into solution. Retest afterwards and rebalance, in the order our pH and alkalinity guide insists on: alkalinity first, then pH.

It works best with the water still. Circulation dilutes it away from the stain, which is inconvenient because it means turning the pump off for a period. Follow the product’s own instruction rather than a general rule.

It treats the pool, not the patch. Whole-pool dosing is the normal method, which is a point in its favor compared with scrubbing a single spot — you cannot reach most of a liner anyway.

Where the metal came from

This is the part that decides whether you do this once or every year.

Well water. By far the most common source. Iron and manganese come out of the ground dissolved and oxidize once they meet chlorine. If you filled from a well, expect this — and treat it at fill time rather than after the stain appears.

Corroding fittings. Ladder feet, screws and anything steel that is not stainless. A rust mark directly under a ladder foot is not a water problem, it is a hardware one.

Copper from a heater or a cell. Less common on a seasonal pool, and worth knowing about because copper staining is teal rather than brown and gets misdiagnosed as algae constantly.

Taylor Technologies’ work on the saturation index is the technical background here: what stays in solution and what deposits is a function of pH, alkalinity, calcium hardness and temperature together, not any one of them. Our calcium hardness guide covers the same interaction from the scaling side.

Organic stains: the free fix

If the tablet test lightened the stain, you do not need to buy a stain product at all.

Brush the area, shock the pool, and run the filter. Organic staining is deposited organic matter and chlorine dismantles it — the same mechanism our algae guide relies on. Brushing first matters here for the same reason it matters there: it exposes the deposit so the chlorine can reach it, and our brush roundup covers the nylon-only rule for vinyl.

The usual cause is something that sat on the liner too long — leaves in autumn, a dead insect, a berry. Our autumn leaves guide is the prevention, and daily skimming is most of it.

What no product fixes

Vinyl that has gone brittle and chalky at the water line is not stained, it is finished. That band gets sun, oils and chemical film concentrated on it all season, and no remover brings it back. Our liner guide covers what does slow it, and our liner replacement guide prices the alternative.

Knowing that boundary is worth as much as the products on this page. People spend real money on stain removers trying to fix age.

What we are scoring

We have not tested these and do not claim to. The scores are our reading of published composition and stated surface compatibility — active ingredient, vinyl safety and what each product targets — against a vinyl-lined seasonal pool. Our methodology page explains how our scoring works.