How to get rid of black algae in a pool

Black algae is a hardy type of algae, really a cyanobacteria, that shows up as raised dark blue or black spots from pinhead to coin size. It grows in grout lines, rough plaster, shady corners and steps. The top layer is tough enough to shrug off normal chlorine, and the roots grow into the pores of the plaster, which is why it comes back.

It gets carried in, then takes hold where chlorine is weak. Swimsuits, floats and toys used in lakes, canals or the ocean can bring it in. It takes hold where chlorine runs low, circulation is poor, or the plaster is rough enough to give it somewhere to grip. Older plaster is especially prone to it.

Break the protective layer, then hit it with chlorine, and repeat. Brushing is not optional with black algae. Shock without brushing kills the top and leaves the roots.

• Brush first. A stainless steel brush on plaster, a nylon brush on vinyl or fiberglass.
• Put chlorine directly on the spots. On plaster, many pool techs rub a chlorine tablet straight onto each spot after brushing. Never do that on vinyl or fiberglass.
• Shock hard after dark with the pH lowered to about 7.2, and run the filter around the clock.
• Brush the spots every day for at least a week.

The numbered steps are below. Treat the chemicals with respect while you do it: the CDC is clear that pool chemicals go into the water one at a time and are never mixed.

Usually one to two weeks of daily brushing and steady high chlorine. The spots often fade in a few days, but if you stop then, the roots grow back. Keep going for a week after the last spot disappears. If the whole pool has turned green on top of the black spots, a green pool cleanup clears the water first.

Not while you are treating it. The chlorine will be far above swimming level, and a pool losing to algae is a pool where the chlorine is not keeping up. Swim again once free chlorine has dropped back to 5 ppm or lower, which is covered in how long after shocking can you swim. And wash any swimsuits used in it before they go in another pool or lake.

Not with the pool full, and pressure washing drained plaster is a good way to damage it. In a drained pool, black algae that has rooted deep is a job for an acid wash, which takes off the thin surface layer the algae lives in. That is why an acid wash is often the clean break for a pool that has fought black algae for years.

Steady chlorine, regular brushing and clean gear.

• Keep chlorine steady. At least 1 ppm, or 2 ppm with stabilizer, per the CDC.
• Brush the walls, steps and corners every week, especially the shady ones.
• Wash swimsuits and toys that have been in a lake, canal or the ocean.
• Keep the filter clean and the pump running long enough. See how often to clean a pool filter.

Brushing the walls and steps is part of every weekly pool service visit for exactly this reason.

Yes. A salt water pool is still a chlorine pool, and black algae takes hold anywhere the chlorine dips or the circulation misses a corner. Salt pools get it too, often when the cell output drops in summer. More in salt water vs chlorine.