How to Get Rid of Black Spot Algae in Your Pool (For Good)

Black spot algae is the most stubborn organism you’ll ever find in a pool. If you’re reading this you’ve probably already tried shocking, brushing and chemical treatments, and the spots keep coming back. There’s a reason for that — and it has nothing to do with how much chlorine you’re adding. This guide explains exactly why black spot algae in your pool is so hard to kill, what actually works, and how to keep it gone for good.

What black spot algae actually is

Black spot algae (often a form of blue-green algae from the cyanobacteria family) isn’t sitting on your pool surface. It’s living inside it. The roots penetrate microscopic pores in concrete, pebblecrete, plaster and aged fibreglass gelcoat. What you see as a black or dark green spot is the visible head of an organism that’s embedded 2 to 5 mm into the surface.

That’s why surface brushing and shocking doesn’t kill it. You’re hitting the tip of the plant while the roots stay protected inside the substrate, and as soon as your chlorine drops back to normal levels the visible spots come back.

Why black spot algae in your pool keeps coming back

The surface is compromised

Black spot thrives in pools with worn, porous or micro-cracked surfaces. Old pebblecrete, chalked fibreglass and cracked plaster all give the algae thousands of microscopic hiding spots. If your pool has had black spot for more than one season, the surface itself is almost certainly part of the problem.

The water chemistry is swinging

Consistent low-level chlorination kills black spot slowly but reliably. Massive chlorine spikes followed by low chlorine periods (the typical Australian pool owner pattern) lets the algae recover between treatments. Stabiliser (cyanuric acid) levels over 80ppm also reduce the effectiveness of chlorine dramatically.

The filtration isn’t working hard enough

Black spot spores need to be filtered out of the water continuously. If your filter is undersized, blocked or only running 4 hours a day, spores keep recirculating and finding new homes on the pool surface.

What actually kills black spot algae

Here’s the real protocol. It’s more work than what you’ll read on most pool shop shelves, but it actually works.

Step 1: Balance water chemistry first

• pH to 7.2 (lower end so chlorine is more active)
• Total alkalinity 80 to 120 ppm
• Calcium hardness 200 to 400 ppm
• Stabiliser below 50 ppm (if over 80, partial drain and refill)

Step 2: Brush aggressively with a steel-bristle brush

This is non-negotiable. Plastic brushes don’t break the protective coating on the algae heads. Use a steel brush (on concrete and pebblecrete only — not fibreglass) to physically scrub every visible spot until the surface is raw.

Step 3: Triple shock with granular calcium hypochlorite

Raise free chlorine to 20+ ppm and hold it there for 48 to 72 hours. Pool shop shock (sodium dichlor) is not strong enough for serious black spot. Liquid chlorine also works but you need a lot of it.

Step 4: Algaecide specifically for black spot

Use a copper-based or silver-based algaecide designed for black algae. These metals bond to the algae cell walls and disrupt reproduction in a way chlorine alone can’t.

Step 5: Run the filter 24/7 for the duration

Backwash or clean the filter media daily while treating. Spores you kill need to be filtered out, or they settle and reinfect.

Why it still comes back (and when resurfacing is the real answer)

Even a textbook treatment sometimes fails to permanently remove black spot because the root system is protected inside the pool surface. If your pool is more than 15 years old, has visible surface wear, or has had recurring black spot for 2+ seasons, chemical treatment alone rarely solves it.

The only permanent fix in that scenario is resurfacing. When we strip off a worn pool surface, the embedded black spot network goes with it. A fresh, non-porous surface denies the algae a foothold. We’ve seen pools that had been battling black spot for 8 years go completely clear within a week of resurfacing — and stay clear.

Read our rough pool surface article for more on how surface wear creates perfect conditions for algae.

Preventing black spot algae coming back

• Keep free chlorine between 2 and 4 ppm consistently, not in spikes
• Run the filter 8+ hours a day during warm months
• Brush the pool weekly, even when it looks clean
• Test stabiliser every 6 months and partial drain if over 80 ppm
• Address surface wear before it becomes a sanctuary for algae

For a proper assessment of whether your surface is beyond chemical control, check our pool condition checker or review the pool resurfacing cost guide. Our pool resurfacing service page covers our exact process for eliminating black spot permanently.

Tired of fighting the same spots every summer? Book a free assessment or call 1800 724 683. We’ll tell you honestly whether chemistry will fix your problem or whether resurfacing is the only real answer.