Black spot in pool: causes, treatments & the real fix

What is black spot in a pool?

Black spot is the common name for a species of cyanobacteria — often called blue-green algae — that forms small, dark, round or irregular colonies on pool surfaces. The dots are usually 3–10mm across, though they can grow larger. They typically show up on walls and floors in darker corners first, but in neglected pools they can spread across the whole surface.

Unlike green algae (which floats freely in your water and responds well to chlorine), black spot anchors into the pool surface. It sends root- like structures down into microscopic pits and pores, and the colony grows a protective polysaccharide layer that shields it from chemicals. That's why the usual “shock and brush” routine only takes the top off — the roots survive, and within a week or two the colony is back in the same spot.

Why does black spot target older pools?

Black spot loves porosity. A brand-new pebblecrete or gelcoat surface is dense, smooth, and gives the algae almost nothing to grab onto. As a pool ages, the surface slowly breaks down:

• Pebblecrete/plaster loses the cement paste between the aggregate, exposing tiny pits and creating a rough, porous microtexture.
• Fibreglass gelcoat develops microcracking, chalking and occasional osmosis blisters that break the surface seal.
• Painted pools develop pinhole failures and chalking that give algae perfect cracks to root into.

Once that happens, the surface itself becomes the problem. Chemicals can only scrub the outside of the colonies. The roots inside are protected, and they regrow. We see this pattern constantly in Adelaide pools in the 15–40 year age bracket — especially older pebblecrete pools that have never been resurfaced.

How to tell it's really black spot

A few lookalikes to rule out first, because treatment is different for each:

• Black spot— round or irregular dark spots, firmly attached, won't brush away easily, leaves a faint outline even after brushing.
• Mustard algae — yellow-brown powder that does brush away easily, often confused with dirt or pollen.
• Metal staining— looks like a stain, not a colony. Usually uniform, doesn't change size over time, often follows water flow patterns (see our pool stains guide).
• Pitting corrosion — small dark pits but no slime layer; feels like pinpricks under a fingertip rather than raised colonies.

If you brush the spot hard and it leaves a faint dark mark that refuses to go away, it's almost certainly black spot rooted into the surface.

DIY treatments: what actually helps

We're going to be honest — if the surface is sound and the outbreak is early, you can beat black spot with elbow grease and chemistry. If the surface is old and pitted, you're only buying time. Here's the sequence we'd try first, in order:

1. Balance your water. Check pH (7.2–7.4), total alkalinity (80–120 ppm), calcium hardness (200–400 ppm) and stabiliser/CYA (30–50 ppm). Unbalanced water feeds algae and degrades your surface faster.
2. Brush the affected areas hardwith a stiff nylon brush for pebblecrete/plaster, or a soft brush for fibreglass. You're trying to physically open the protective slime layer so chemicals can reach the colonies.
3. Shock the pool to 20ppm chlorineand hold it for 48–72 hours. Standard shock doses won't penetrate — you need sustained high chlorine.
4. Add a quality algaecide rated for black spot. Follow dosing instructions to the letter — overdosing creates foam and staining.
5. Brush again every 6–12 hours for the first three days. This is the most important step and the one most people skip.
6. Run the filter 24 hours a day for a week and backwash or clean cartridges as soon as they load up.

Expect visible improvement within 48 hours if it's going to work. If the spots fade but return within a few weeks, the surface is compromised and no amount of chemistry will hold them back permanently.

The limits of chemical treatment

Here's why chemicals can't always win. Black spot grows inside the surface layer, and once a colony is established:

• The slime coatingresists chlorine until it's physically disrupted — which is why brushing matters more than dosing.
• The root structures penetrate 1–3mm into porous surfaces, further than any chemical can reliably reach.
• Every outbreak slightly erodes the surface, making the next outbreak easier. It's a one-way slope.
• Copper-based algaecides can stain the poolpermanently if overused — which is ironic because you're treating dark spots with more dark spots.

The only permanent fix: full resurfacing

When we resurface a pool, we physically grind or sandblast off the failed surface layer — and the black spot roots go with it. Then we apply a fresh, dense, bonded coating (pebblecrete on concrete, or a new gelcoat on fibreglass) that gives algae nothing to root into. Our customers who used to get black spot every single summer stopped getting it entirely after the resurface.

The difference is massive. A properly prepared and resurfaced pool is 15–25 years of hassle-free water. A pool where black spot has won is a constant maintenance battle and eventually a forced fix anyway — usually in a worse state than if you'd addressed it earlier.

We offer concrete pool resurfacing for pebblecrete and rendered concrete pools, and full pool resurfacingacross fibreglass and concrete. Every project starts with a free on-site assessment so we can see exactly what we're dealing with and give you honest options.

How to prevent black spot from coming back

Once your pool is resurfaced, prevention is straightforward. The big three are:

• Keep water balanced — pH and alkalinity are the usual culprits when things go wrong.
• Brush shaded areas weekly — steps, corners under overhangs and the shallow end. Disturbance prevents establishment.
• Maintain free chlorine at 2–4ppm year-round, even in winter when pools sit idle and water gets cold.

Adelaide pools sit through long hot summers and cool, still winters — both create opportunities for algae. A maintenance routine that accounts for both seasons keeps a fresh surface looking new for decades.

Related problems to watch for

If you have black spot, there's a good chance you also have one of its neighbours. A surface that's let black spot root into it is usually also showing signs of:

• Rough pool surface — the same porosity that lets black spot root also makes the surface feel like sandpaper.
• Pool stains — aged surfaces pick up metal and organic stains much more easily than new ones.
• Surface pitting and small chips that turn into bigger problems if left.

When to call us

Call Blue Revive if any of the following sound familiar:

• You've had two or more outbreaks in the same season
• You can still see the ghost of the spots even after treatment
• The pool surface feels rough or sandpapery to touch
• The pool is 15+ years old and has never been resurfaced
• You're spending more than $100/month on chemicals in a losing battle

We'll assess the surface on-site, give you a frank opinion on whether it can be saved with treatment or whether resurfacing is the right call, and — if resurfacing is the answer — walk you through exactly what we'd do and what it would cost. See our pool resurfacing cost guide for current Adelaide pricing before you call.