Symptom Library
Algae or Stain? How to Identify Any Mark in a Fiberglass Pool
Green fuzz, brown shadows, black dots, before you buy a single chemical, find out what you're actually fighting. Three simple tests separate algae from stains from metal from surface failure, and one lookalike fools even pool-store veterans.
The fast way to tell algae from a stain: algae is alive, so it responds, it brushes off, smears, and dies back when you shock the pool. A stain is chemistry bonded to the surface, so it ignores the brush and ignores chlorine. Metal stains have their own tell: they lift under a vitamin C tablet. And the marks that respond to nothing at all usually aren't on the surface, they're the surface itself failing. Run the three tests below in order and you'll know which one you have in about ten minutes.
Test 1: The Brush Test
Take a nylon pool brush, never wire on fiberglass, it scratches gel coat, and scrub the mark hard.
- It smears, clouds the water, or wipes away: algae or another organic film. Living growth sits on top of the surface and gives way to friction. Green and yellow (mustard) algae usually surrender to a brush easily; black algae resists because it roots, but the head of the colony will slough.
- Nothing moves: it's a stain or a surface problem. Stains are compounds deposited into or bonded onto the gel coat, no amount of brushing relocates them.
One fiberglass-specific note: gel coat is non-porous, which is good news. Algae and stains both have a harder time anchoring to fiberglass than to plaster, so marks that would be permanent in a gunite pool often clean up completely on a healthy gel coat. The key word is healthy, more on that below.
Test 2: The Chlorine Test
Algae responds to sanitizer; stains don't. Two ways to run it:
- The spot version: hold a chlorine tablet (in a tablet holder or with gloves, don't rest bare trichlor on gel coat for long) directly against a small mark for 30-60 seconds. Organic material, algae, leaf tannin, body oils, bleaches visibly pale. Metal stains and surface defects don't budge, and some metal stains actually darken.
- The whole-pool version: shock the pool and brush daily for 2-3 days. Algae dies, turns gray-white, and filters out. A stain looks exactly the same after the shock as before it.
This one test settles most green and yellow mysteries: green algae clears with shock, while a green-tinted stain (often copper) laughs at it.
Test 3: The Vitamin C Test for Metal
Hold a plain vitamin C (ascorbic acid) tablet against the mark for 60 seconds, or rub it gently across the spot. If the mark visibly lightens or lifts, you've got a metal stain, classically iron, which stains brown to rust-red. Copper leans green-blue to gray-black and responds better to citric acid, but ascorbic will usually still lighten it enough to confirm metal.
Metal stains come from fill water, corroding equipment (a failing heat exchanger is a classic copper source), well water top-ups, or metal objects left on the surface. The fix is a stain-removal treatment plus a sequestrant, and finding the metal's source, or the stain returns. The full removal playbook is in how to clean fiberglass pool stains.
The Identification Table
| Mark color | If it's algae | If it's organic stain | If it's metal | If it's surface failure |
|---|---|---|---|---|
| Green | Slimy film, brushes off, dies with shock, green algae | Leaf/plant tannin shadow under debris; bleaches with chlorine spot test | Copper: green-blue tint, ignores shock, responds to acid | Rare, but algae regrowing in the same rough patch means the gel coat there is failing |
| Yellow | Mustard algae: dusty sheets on shaded walls, brushes off, needs aggressive shock | Pollen film along the waterline; skims and filters out | Early iron can read yellow-brown; vitamin C lifts it | Yellowed gel coat from age or chemistry burns, even, doesn't wipe |
| Black | Black algae: raised, rooted dots that resist brushing; heads slough with effort | Uncommon, decayed debris resting marks; bleach and brush out | Copper or silver can deposit gray-black; acid test responds | Cobalt spots, blisters erupting from inside the laminate; nothing topical works |
| Brown | Some algae reads brown, but it brushes and shocks away | Tannin from leaves and acorns: tea-colored shadows; chlorine bleaches | Iron: rust-brown patches or streaks; vitamin C lifts them | Exposed laminate showing through worn gel coat, permanent, and a resurfacing sign |
The One That Fools Everyone: Black Algae vs Cobalt Spots
Here's the misdiagnosis we see constantly on fiberglass pools. Black dots appear, the pool store says "black algae," and the owner spends a season scrubbing and shocking a problem no chemical can touch, because those dots were never alive.
Cobalt spots are a fiberglass-specific defect: tiny blisters born inside the laminate itself, where the resin didn't fully cure at the factory. Water works into the flaw, reacts, and erupts through the gel coat as a small black-rimmed pit. They're coming from underneath the surface, which is why they shrug off every treatment applied on top of it.
How to tell them apart in ten seconds: black algae is a raised bump that feels slightly slick and sloughs under hard brushing; a cobalt spot is a pit or crater, often with a tiny blister dome or rust-black halo, and brushing does nothing, or pops it into a bigger pit. Algae multiplies fastest in shade and low circulation; cobalt spots scatter wherever the factory flaws happen to be. If your black dots pit rather than bump, read black spots in a fiberglass pool, that's a surface problem, not a water problem.
If your marks failed all three tests, won't brush, won't bleach, won't lift with vitamin C, stop buying chemicals. You're not looking at something on your pool; you're looking at your pool. Cobalt spots, exposed laminate, and chemistry-burned gel coat are surface failures, and the cure is gel coat resurfacing: grind out the defects, repair the laminate, and spray a factory-grade marine gel coat that seals the whole shell for 15-25 years. Send us photos of the marks, close-up and wide, and a specialist will tell you within one business day whether you need a treatment or a treatment plan. Free either way.
Why Identification Matters More on an Aging Surface
One pattern worth naming: recurring algae and stubborn staining are often symptoms of the surface, not the water. Healthy gel coat is smooth and non-porous, hostile territory for algae roots and stain deposits. As gel coat oxidizes and wears, it turns microscopically rough and porous, and suddenly the same water chemistry that kept the pool spotless for a decade can't keep up. If you're fighting marks in the same spots every season, and those spots feel rough to the touch, the marks are messengers. Treat the message, not just the mark.
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Straight Answers
Algae and stain questions
How do I tell if it's algae or a stain in my pool?
Brush it, then shock it. Algae is alive: it smears under a brush and dies back after shocking. A stain ignores both. If a chlorine tablet held against the mark bleaches it, it's organic; if a vitamin C tablet lifts it, it's metal; if nothing touches it, suspect the surface itself.
What are the black spots in my fiberglass pool that won't scrub off?
If they're pits or tiny blisters rather than raised slick bumps, they're likely cobalt spots, a defect inside the fiberglass laminate, not black algae. No chemical or brushing fixes them because they erupt from beneath the gel coat. They're repaired by grinding out the flaw and resurfacing.
Does the vitamin C test damage a fiberglass pool?
No. Holding or rubbing an ascorbic acid tablet on a spot for a minute is harmless to gel coat, it's the standard metal-stain test. If the mark lightens, treat it as an iron or copper stain and track down the metal source so it doesn't come back.
Why does algae keep coming back in the same spots?
Usually because those spots give it grip. Worn, oxidized gel coat is rough and porous where healthy gel coat is smooth, so aging areas re-grow algae even with good chemistry. If the recurring spots feel rough to the touch, the surface, not your sanitizer, is the problem to fix.
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