That Stain on Your Pool Floor Isn't Algae. Here's What It Actually Is.

Your pool water is clear, your chemistry looks balanced, but there's a brownish stain spreading across the shallow end. You shock the pool. The stain gets darker. If that sounds familiar, you've made the most common pool maintenance mistake: treating a stain like it's an algae problem.

Pool stains fall into three categories — metal, organic, and surface deterioration — and each requires a completely different fix. Get it wrong and you don't just waste money; you can drive a stain deeper into your finish. Here's how to read what you're looking at and treat it correctly.

Metal stains account for the majority of staining calls we see. They look alarming — deep brown, rust-colored, or greenish-black marks that can appear suddenly after shocking the pool. The reason they get worse after treatment: chlorine oxidizes dissolved metals, causing them to precipitate out of solution and embed into your plaster or pebble finish.

The most common culprits: Iron produces brown, rust, or yellow staining — often in streaks or irregular splotches. Copper leaves blue-green or teal marks, frequently from corroded heat exchanger components or copper-based algaecides. Manganese shows up as purple, black, or dark brown and is especially common in well water and some municipal supplies across the Carolinas.

To confirm a metal stain, place a small amount of ascorbic acid powder — plain vitamin C — directly on the stain. If it lightens within 30 seconds, you're dealing with metal. For copper specifically, a muriatic acid spot test gives you the same confirmation in about the same amount of time.

Treatment for iron and copper stains starts with an ascorbic acid dose: roughly 1 lb per 10,000 gallons, circulated without chlorine for about 30 minutes. The acid lifts metal off the surface and brings it back into solution. Follow that immediately with a sequestrant — a chelating agent that binds metals and prevents them from reattaching. A full treatment typically runs $50–$120 depending on pool size and severity.

Prevention matters more than treatment. If you fill with well water, use copper-based algaecides, or have aging equipment, you're adding metals continuously. A monthly sequestrant product ($15–$30/month) keeps metals suspended so they flow out through your filter rather than landing on your finish.

Organic stains come from leaves, berries, grass, pollen, and other plant material that breaks down on your pool surface. If you've ever pulled a wet leaf off plaster and found a perfect brown outline underneath, you know exactly what this looks like.

These stains are almost always surface-level, and chlorine is your friend. Placing a trichlor tablet directly on the stain and letting it sit for 30–60 minutes bleaches most organic marks without additional treatment. For larger affected areas, a pool enzyme product — not the same thing as shock — used weekly prevents buildup from accumulating in the first place.

The diagnostic shortcut: organic stains are brown, tan, or greenish-brown, and they tend to mirror the shape of whatever caused them. A rust-colored streak that runs along a crack or around a fitting is almost certainly metal, not organic matter.

If staining is widespread, returns quickly after treatment, or lives in pitted or rough patches, the issue may not be your water chemistry at all — it's the surface. Plaster pools older than 10–15 years develop calcium scaling, etching, and porosity that makes them nearly impossible to keep clean. Pebble and quartz finishes last longer, typically 15–25 years, but calcium nodules and deposits still accumulate along waterlines and in low-flow zones.

For surface calcium scale, a wet pumice stone removes deposits from tile and coping without damaging the underlying material. For widespread scaling across plaster, an acid wash is often the right call — it removes the degraded top layer and exposes fresh surface below. Expect to pay $300–$600 in the Carolinas for a professional acid wash, and it can add several years of useful life to a finish that would otherwise need full resurfacing.

Persistent staining almost always traces back to imbalances held too long. Low pH (below 7.2) makes water aggressive — it leaches metals from equipment and attacks plaster, setting up both metal and surface staining fast. High calcium hardness (above 400 ppm) drives calcium out of solution and onto your surfaces. If total dissolved solids have crept above 1,500 ppm in a chlorine pool, a partial drain-and-refill often resolves staining that months of targeted chemical treatment couldn't eliminate.

The fix starts with your test kit, not the store shelf. Get an accurate water test — a full panel, not just a chlorine strip — before you buy anything.

Run the vitamin C test first. It costs nothing, takes 30 seconds, and tells you immediately whether you're dealing with a metal problem or something else. That single data point determines the entire treatment path and keeps you from buying products that won't work.

If you've treated the same stain multiple times and it keeps coming back, that's typically a sign of a structural chemistry issue or surface deterioration that needs a more experienced eye. The Rock Water Pools team has worked through every combination of stain, finish, and water chemistry that Carolina pools can produce — and we're glad to help you figure out the right fix rather than guess at it. Call us at 704-450-1023, and let's talk through what you're seeing.