What is the Vitamin C test for pool stains?

The Vitamin C test is a diagnostic trick used to identify metal stains. Place a handful of crushed, cheap Vitamin C tablets (Ascorbic Acid) inside an old sock and rub it directly on the pool stain. If the stain vanishes or lightens instantly, it is a metal stain (usually iron). If nothing happens, it is likely an organic stain.

How do you remove organic stains from a pool?

Organic stains (caused by leaves, acorns, or dead algae) are easily removed by elevating the Free Chlorine levels. You can test this by holding a standard 3-inch Trichlor tablet directly on the stain for 30 seconds. If it bleaches away, it is organic. To clear the whole pool, brush the surfaces and perform a SLAM (Shock Level and Maintain) treatment.

Pool Stain Identifier: Metals vs. Organics | Pool Checker

Pool Stain Identifier: How to Diagnose Metals vs. Organics

Pouring random chemicals into your pool to clear a stain is expensive, frustrating, and often dangerous. In fact, if you shock a pool that has a metal stain, you can instantly oxidize the metal and turn the entire pool neon green or permanently stain the plaster black. Before you treat a stain, you must definitively prove whether it is Organic (caused by algae or leaves) or Inorganic (caused by iron or copper).

A pool owner inspecting a brown stain on the bottom of a swimming pool — Never shock a pool with mysterious brown or black stains without testing them first.

The 2 Golden Rules of Stain Diagnosis

Pool professionals don't guess what a stain is; they use chemical reactions to prove it. You can perform these exact same tests using items you likely already have in your house.

Test 1: The Vitamin C Test (For Metals)

Ascorbic acid (Vitamin C) is a powerful reducing agent that instantly lifts metal stains off pool plaster. Crush up a handful of cheap Vitamin C tablets, put them inside a thin white sock, and hold it directly against the pool stain for 30 seconds. If the stain vanishes or turns significantly lighter, it is a metal stain.

Test 2: The Trichlor Test (For Organics)

If the Vitamin C did nothing, the stain is likely organic. To prove it, take a standard 3-inch chlorine tablet (Trichlor) and hold it directly on the stain for exactly 30 seconds (do not leave it longer or it will etch your plaster). If the stain bleaches away and disappears, it is organic matter.

Interactive Stain Diagnostic Wizard

Perform the tests above, select your stain color, and let our engine generate your exact chemical treatment protocol.

The Stain Color Matrix

While the chemical tests above are the only way to be 100% certain, the color of your pool stain gives you a massive clue about the underlying cause.

Brown, Red, or Rust Colors
Almost always Iron. Usually introduced by filling the pool with well water, using cheap lawn fertilizers near the pool, or severely corroded heater headers.
Teal, Blue, or Purple Colors
Almost always Copper. Typically caused by overusing cheap copper-based algaecides, or maintaining a very low pH which strips copper from your heat exchanger.
Green or Brownish-Black Smudges
Almost always Organic Tannins. Caused by crushed leaves, acorns, or localized algae blooms settling on the floor and embedding into the plaster.

Frequently Asked Questions

Why did my pool turn green when I shocked it?

If your pool water turns neon green, teal, or purple immediately after adding chlorine shock, you have high levels of dissolved copper in the water. The chlorine oxidized the copper, turning it into a visible suspension. You must use a metal sequestrant to bind the copper so your filter can remove it.

Do I need to drain my pool to remove a metal stain?

Not always. You can lift the stain off the plaster using Ascorbic Acid (Vitamin C), but the metal will still be floating invisibly in the water. To prevent the stain from returning, you must use a sequestrant (like CuLator) to trap the metals in the filter. If the metal levels are astronomically high, a partial drain is the only permanent solution.

Need to Treat an Organic Stain?

Time to SLAM the Pool

If the Trichlor test proved your stain is organic, you need to elevate your chlorine levels to aggressively bleach the plaster clean. Use our calculator to find your exact shock dosage.

👉 Calculate Shock Target