Black Algae Treatment — The Colony TX

Green algae is a nuisance. Yellow algae is stubborn. Black algae is a different category entirely — it's the one type of algae that makes experienced pool technicians pause, because killing it requires more than chemistry. It requires physically destroying the organism's protective structure, and if you miss even a small colony, it grows back from the survivors within weeks.

If you're seeing small, dark blue-black spots on the plaster of your Colony pool — particularly in shaded areas, on the steps, or in corners with low circulation — you're dealing with cyanobacteria, commonly called black algae. It's more common in pools near Lewisville Lake (Austin Waters, Stewart Peninsula, The Tribute) because the lake's ecosystem harbors cyanobacteria that become airborne and settle into nearby pools.

Why Black Algae Is Different From Every Other Pool Algae

It Has Roots

Green algae and yellow algae float in the water column or form a surface film on pool walls. You can brush them off and the chlorine in the water kills what remains. Black algae doesn't work like this.

Black algae forms a nodule — a raised, dark spot that anchors into the porous surface of plaster or gunite. Beneath the visible nodule, root-like filaments called rhizoids penetrate into the plaster substrate. These roots extend into the material itself, not just on the surface. When you brush the visible nodule off, the roots remain embedded in the plaster — and they regrow the nodule within days.

This is why black algae "comes back" after treatment. You didn't fail to kill it. You killed the visible part and left the invisible part alive inside your pool surface.

It Has a Protective Cap

The surface of a black algae nodule is covered by a waxy, layered cap that's highly resistant to chlorine penetration. Normal pool chlorine levels (2-3 ppm) can't break through this cap. Even shock levels (10-15 ppm) struggle to penetrate it. The cap is the organism's defense mechanism — it evolved to survive in harsh environments, and your pool's chemistry qualifies as a harsh environment that it's specifically adapted to resist.

This is why "just shock it" doesn't work for black algae the way it works for green algae. Green algae has no protective barrier — chlorine contacts and kills it directly. Black algae's cap deflects chlorine the way a raincoat deflects rain. The chlorine rolls off the surface without penetrating to the living cells underneath.

It Photosynthesizes Through Pool Plaster

Black algae can survive in surprisingly low-light conditions. The cyanobacteria cells within the nodule photosynthesize even through the slightly translucent plaster surface. This means they can establish in areas that don't receive direct sunlight — shaded walls, under pool steps, in corners that the sun never reaches directly.

This also means that blocking sunlight (covering the pool) doesn't kill established black algae the way it might slow green algae growth. The organisms adapt to reduced light and continue surviving.

How to Actually Kill Black Algae

The process requires physical destruction of the protective cap followed by chemical penetration to the roots. Neither step alone is sufficient. Both steps must happen in sequence, and the process must be repeated to catch any colonies you missed.

Step 1: Aggressive Brushing With a Stainless Steel Brush

Not a nylon brush. Nylon bristles are too soft to break through the waxy cap. You need a stainless steel bristle brush — the kind with metal wire bristles that feels like you're scrubbing with steel wool.

Brush each individual nodule directly. Don't just sweep across the wall — target each dark spot and scrub it specifically with firm pressure. The goal is to physically break through the protective cap and expose the living cells underneath to the pool water (and the chlorine in it).

This step is labor-intensive. A pool with 50 black algae spots requires you to individually scrub each one. If the spots are on the floor and steps, you'll be in the pool doing it. Allow 30-60 minutes for a thorough brushing session.

After brushing, the spots will look lighter — you've removed the dark cap material and exposed the green/blue living tissue underneath. The water will be cloudy with brushed-off debris. That's expected.

Step 2: Apply Granular Chlorine Directly to Each Spot

Trichlor tablets or granular cal-hypo applied directly to the brushed nodule. The exposed cells — now without their protective cap — are vulnerable to concentrated chlorine contact.

Method for walls: Take a trichlor tablet and hold it against the brushed spot for 30 seconds, rubbing it in small circles. The tablet dissolves slowly, releasing concentrated chlorine directly onto the exposed algae. Do this for every spot on the walls.

Method for floor and steps: Sprinkle granular cal-hypo directly onto the brushed spots. Let it settle onto the surface. The granules dissolve slowly, maintaining a concentrated chlorine zone on the spot for several minutes. Turn the pump off during this step so the granules aren't circulated away before they dissolve on the target.

Alternative: Some pool professionals use a chlorine paste (cal-hypo mixed with a small amount of water to form a thick paste) applied directly to each spot with a putty knife. This maintains contact longer than dry granules.

Step 3: Shock the Pool to Maximum Chlorine Level

After the direct treatment, shock the entire pool to 25-30 ppm free chlorine — triple shock dose. This sustained high chlorine level continues penetrating into the plaster where the roots remain.

Use liquid chlorine for the shock — you've already added cal-hypo directly to the spots, and additional cal-hypo raises calcium hardness unnecessarily. Liquid chlorine is the cleanest shock option.

Maintain the elevated chlorine level for 72 hours. Retest every 12 hours and add more liquid chlorine as needed to keep FC above 20 ppm. The roots deep in the plaster need sustained chemical exposure over days, not hours.

Step 4: Run the Pump 24/7

Continuous circulation ensures the shock-level chlorine reaches every surface of the pool consistently. Don't reduce pump run time during black algae treatment — run it around the clock until the treatment is complete.

Step 5: Clean the Filter Daily

Dead black algae cells, brushed-off cap material, and general debris from the treatment load the filter heavily. Check filter pressure every 12 hours and clean (rinse cartridge, backwash sand/DE) when pressure rises 8-10 psi above baseline. A clogged filter during treatment reduces circulation and compromises the chemical distribution you need.

Step 6: Brush Again on Day 2 and Day 3

Repeat the aggressive stainless steel brushing on days 2 and 3. Even after the initial treatment, some nodules will still be visible — the roots survived the first round. Breaking the regenerating cap material exposes the cells to the still-elevated chlorine in the water.

By day 3, the treated spots should appear white or bleached — that's dead algae tissue. Dark spots that haven't lightened still have living cells and need additional direct chlorine contact.

Step 7: Inspect on Day 5-7

After 5-7 days of treatment (sustained high chlorine, repeated brushing, continuous filtration):

All spots are white/bleached and no longer raised: Treatment successful. The black algae is dead. Lower chlorine back to normal levels (2-3 ppm), rebalance pH and alkalinity, and return to normal maintenance.
Some spots are still dark or have regrown their cap: The roots in those specific spots survived. Repeat steps 1-3 on the surviving spots only. A second round usually eliminates the holdouts.
New spots appearing in areas that were previously clean: The treatment killed the visible colonies but spores in the water established new ones. This is rare if chlorine was maintained above 20 ppm for the full treatment period — but it can happen. A full second treatment round is needed.

Why It Comes Back (And How to Prevent It)

Reason 1: Incomplete Treatment

The most common reason. If the brushing didn't break through every nodule's cap, or if chlorine levels dropped below 15 ppm during the treatment period, surviving cells regrow within 2-4 weeks. The treatment must be thorough and sustained — half-measures fail.

Reason 2: Contaminated Pool Toys or Equipment

Black algae spores survive on pool toys, floats, brushes, and equipment that were in the pool during the outbreak. If you treat the pool but don't sanitize the accessories, the spores reintroduce the organism when you put the toys back.

During treatment: Remove all toys, floats, and non-essential equipment from the pool. Soak them in a chlorine solution (1 gallon liquid chlorine in a trash can of water) for 1 hour before returning them to the treated pool.

Reason 3: Deteriorated Plaster

Black algae establishes more easily in rough, pitted, or deteriorated plaster because the textured surface provides more anchor points for the rhizoids and more protected crevices where the nodules can establish. If your Colony pool has rough plaster (common in pools 15+ years old), the surface itself makes black algae more likely to establish and harder to kill.

Long-term solution: resurfacing. A fresh plaster surface eliminates the embedded roots and the rough texture that facilitated establishment. If black algae keeps returning despite thorough treatment, the plaster condition may be the underlying enabler.

Reason 4: Persistent Introduction From the Environment

Colony's proximity to Lewisville Lake means cyanobacteria spores are present in the local environment. They enter pools via wind, rain, on swimsuits worn in natural water, and even on shoes. You can't prevent spore introduction entirely — but you can prevent establishment.

Prevention protocol:

Maintain chlorine at 2-3 ppm consistently. Black algae can't establish on a properly chlorinated surface. It exploits lapses — the week you forgot to add chlorine, the vacation where the pool went to zero FC.
Brush walls and floor weekly. Regular brushing disrupts nascent colonies before they can build their protective cap. A colony that's brushed off at day 2 never develops the root structure that makes it hard to kill at day 14.
Maintain pH at 7.2-7.4. Lower pH means more active chlorine available for surface sanitation.
Pay attention to low-circulation areas. Corners, behind ladders, under steps, and shaded wall sections are where black algae establishes first. Direct return jets toward these dead zones and brush them specifically during weekly maintenance.

When to Call a Professional

DIY treatment works for minor outbreaks — fewer than 20 spots, all accessible, on surfaces you can brush with a stainless steel brush.

Call a professional for:

Extensive outbreaks — 50+ spots covering large wall or floor areas. The labor and chemical volume required makes professional treatment more practical.
Black algae on pool steps or in spa jets — these areas are difficult to brush effectively and may require disassembly for complete treatment.
Recurring black algae despite thorough treatment — this suggests a surface condition (deteriorated plaster) or a persistent introduction source that a professional can assess.
If you're not sure it's black algae. Dark spots on plaster can also be mineral staining (iron, manganese, copper) that requires completely different treatment. A professional can distinguish between biological growth and mineral deposits with a simple test — chlorine bleaches algae but doesn't affect mineral stains.

Black algae you can't beat? Hydra Pool Services treats black algae with the full protocol — stainless steel brushing, direct chlorine application, sustained shock, and follow-up monitoring — across The Colony, Frisco, Plano, McKinney, Allen, Murphy, and Parker. Get it treated →

Black Algae in Your Colony Pool? Why It's So Hard to Kill (And How to Actually Do It)