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<\/figure>\n\n\n\nGreen is the common one<\/a> and the least trouble. It clouds the water or forms a film on the walls, lifts under a brush, and dies off readily once the chlorine comes up. <\/p>\n\n\n\n Yellow, the kind people call mustard algae<\/a>, is more stubborn. It clings to the shaded side of the pool in dusty-looking sheets that brush away and return, and it shrugs off a normal chlorine level. <\/p>\n\n\n\n Black algae<\/a> is the one that earns the word stain. It roots down into porous plaster and builds a waxy cap over itself, so a brush skims the top while the roots sit safe underneath, and the spot it held stays marked in the plaster even after the organism is dead.<\/p>\n\n\n\n Nothing lifts an algae stain<\/a> while the algae is alive, so killing it comes first, and that runs on a brush and chlorine working together. <\/p>\n\n\n\n Brushing breaks the surface open and, on black algae, cracks that protective cap so the chlorine can reach the roots at all. <\/p>\n\n\n\n Then the pool gets shocked, free chlorine driven well above the 1 ppm minimum<\/a> a pool holds day to day, with the pH kept in the 7.0 to 7.8 band where chlorine does its best work, and held there with the filter running until the bloom is dead. Brush it again while the chlorine\u2019s high. <\/p>\n\n\n\n Black algae usually asks for more, so use a stiff or stainless brush on the capped spots, and a trichlor tablet<\/a> rubbed straight onto a stubborn patch to put raw chlorine against the roots. <\/p>\n\n\n\n CDC\u2019s instruction for public pools is to scrub the surfaces and take off the slime layer, and that scrubbing is what turns a shock from a water treatment into a wall treatment.<\/p>\n\n\n\n Once the algae is dead, what stays on the plaster is an organic stain, and organic stains answer to chlorine. <\/p>\n\n\n\n A green or brown mark lingering after the water has cleared usually comes up with more of the same: holding the chlorine high, working the spot with a brush, sometimes pressing a chlorine tablet against it. <\/p>\n\n\n\n The one thing worth ruling out is metal. A stain that ignores chlorine entirely points to iron or copper in the plaster rather than algae, and that runs on a completely different treatment.<\/p>\n\n\n\n Algae takes hold where the water goes still, and the chlorine runs thin, like the corners the return jets never reach.<\/p>\n\n\n\n Holding free chlorine up, keeping the water moving, and brushing the walls is most of the prevention right there. <\/p>\n\n\n\n Two things work against it: phosphates feed algae, and cyanuric acid<\/a> weakens chlorine as it climbs, so a pool that keeps blooming on a chlorine reading that looks fine often has a stabiliser level sitting too high underneath.<\/p>\n\n\n\n A brush run over the walls once a week is the single move that keeps algae from ever getting started, and it\u2019s the one that gets skipped. <\/p>\n\n\n\nKill It Before Touching the Stain<\/h2>\n\n\n\n
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<\/figure>\n\n\n\nLift the Stain That\u2019s Left<\/h2>\n\n\n\n
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<\/figure>\n\n\n\nStop It Coming Back<\/h2>\n\n\n\n
Keep the Walls Brushed<\/h2>\n\n\n\n