What causes black algae in Ottawa pools and how do I get rid of it?

Black algae is one of the most stubborn and difficult pool contaminants Ottawa homeowners can face, and it appears as small dark blue-black spots embedded in pool surfaces that resist normal brushing and chlorination because the organism grows roots into porous materials and produces a protective waxy coating that shields its cells from chemical treatment. Getting rid of it requires aggressive, sustained treatment over 7 to 14 days — there is no overnight fix.

Despite its name, black algae is technically cyanobacteria, not true algae. It is a photosynthetic organism that has been on Earth for billions of years and has evolved remarkable survival mechanisms. The "roots" (called rhizoids) penetrate into plaster, concrete, grout, and even the pores of fibreglass surfaces, anchoring the organism so deeply that brushing the visible surface head off does not kill the embedded root structure. The waxy protective cap on each spot repels chlorine and algaecide, which is why a pool can have adequate free chlorine levels and still harbour active black algae colonies. In Ottawa, black algae is most commonly found in concrete and plaster pools, but it can also establish on the textured surfaces of vinyl liner seams, around fittings, and on fibreglass steps.

Black algae enters Ottawa pools through several vectors. The most common source is contaminated swimwear and pool toys that have been used in natural bodies of water — the Ottawa River, Rideau Canal, Meech Lake, and the many quarry swimming holes in the Gatineau Hills all harbour cyanobacteria naturally. A single swimsuit worn in the river and then in your pool can introduce spores. Contaminated fill water from garden hoses that have been sitting in warm sun, bird droppings, and airborne spores carried by wind from nearby waterways are additional sources. Ottawa's location at the confluence of three rivers means cyanobacteria is abundant in the local environment.

Identifying black algae correctly is important because the treatment protocol is different from green or yellow algae. Black algae spots are typically 5 to 15 millimetres in diameter, dark blue-black in colour, and feel slightly rough or raised when you run your fingers over them. They do not brush off easily — in fact, light brushing may appear to remove the surface layer while leaving the root structure completely intact. Green algae, by contrast, is a surface film that brushes off readily and floats into the water column. Yellow (mustard) algae is a fine dusty coating that resettles quickly after brushing. If you see spots that resist brushing and remain anchored to the surface, assume black algae.

The treatment protocol for black algae in an Ottawa pool requires four simultaneous actions: aggressive mechanical scrubbing, extreme chlorine shock levels, a penetrating algaecide, and continuous filtration. Skipping any one of these steps will likely result in regrowth.

Start by scrubbing each black algae spot individually with a stainless-steel brush (for concrete and plaster pools) or a firm nylon brush (for vinyl-liner pools). The goal is to physically break through the waxy protective cap so that chemicals can penetrate to the root structure. For small colonies, a stiff-bristle hand brush or even a chlorine tablet held directly against the spot and rubbed in a circular motion works effectively. This direct contact delivers concentrated chlorine right to the organism. Scrub every visible spot, no matter how small — a single surviving colony will reinfect the entire pool.

After scrubbing, shock the pool to an extreme chlorine level — 30 ppm or higher, which is triple the normal shock dose. For a 50,000-litre pool, this requires approximately 4 to 5 bags (450 grams each) of calcium hypochlorite shock, costing $20 to $50 for the treatment. This level of chlorine is far above the safe swimming range (1 to 3 ppm), so the pool will be closed to swimmers for 2 to 4 days while the chlorine level drops back to normal. Do not be alarmed by the high dose — breaking through black algae's defences requires chemical firepower that normal shock levels cannot provide.

Apply a copper-based algaecide specifically formulated for black algae. Copper algaecide penetrates the protective waxy layer more effectively than quaternary ammonia (quat) algaecides used for green algae. A quality copper algaecide costs $25 to $50 at Ottawa pool supply stores. Follow the dosage instructions precisely — overdosing copper can stain pool surfaces and turn blonde hair green. Typical dosage is 100 to 200 millilitres per 40,000 litres. Apply the algaecide after shocking, not before, so the high chlorine level does not degrade the copper compound before it reaches the algae.

Run your filter system 24 hours a day throughout the treatment period. Dead black algae cells must be physically filtered out of the water, and the high chlorine level needs constant circulation to reach every surface. Backwash or clean the filter every 24 hours during treatment — the filter will collect significant debris. After 3 to 4 days, re-scrub all previously affected areas and re-shock if any spots still show dark colouration. Some stubborn infections require 2 to 3 complete treatment cycles before the black algae is fully eradicated.

Preventing black algae recurrence in your Ottawa pool requires ongoing vigilance. Maintain free chlorine at 3 to 4 ppm (the upper end of normal) rather than allowing it to drift to 1 ppm between treatments. Brush the pool walls and floor twice per week during peak season — especially textured areas where spores lodge. Require all swimmers to shower before entering the pool, and never allow swimwear used in natural water bodies to enter the pool without being washed in a washing machine first. Consider adding a maintenance dose of copper algaecide — typically half the treatment dose — every two weeks throughout the season as a preventive measure. The cost is modest at $50 to $100 for the full season and provides meaningful ongoing protection.

Dealing with black algae in your Ottawa pool? Ottawa Pool Installation connects homeowners with experienced local pool treatment professionals who have the equipment and expertise to eradicate even the most stubborn cyanobacteria infestations.