Nothing spoils the look of summer clothing like black stains. Especially on bathing suits or other clothes worn close to ponds, lakes and swimming pools, those stains may be caused by tenacious black algae. The most difficult algae to remove from both surfaces and fabrics, black algae flourishes in damp or wet areas. Getting rid of algae stains in general may take several applications of cleaners and sometimes more than one kind. Using an enzyme pre-soak followed by either chlorine or oxygen bleach is a good strategy for removing these unattractive stains.
Pre-soak stained fabric in a bowl or other container, following directions on the enzyme pre-soak liquid label. Gently work liquid into stained areas with a plastic scrubber or old toothbrush. Your goal is to expose all fabric threads to the liquid, not scrub the stain out vigorously. Rough handling will likely replace stains with scuff or wear marks. Leave fabric soaking in the liquid for eight hours or overnight before washing. Scrub gently before laundering to see if the stains are dissipating.
Launder stained fabric in a small washer load with oxygen bleach in the quantity prescribed by the label, using cool water to avoid setting any remaining stains. Do not put fabric in the dryer until you are sure that stains are gone; heat can set them permanently. Repeat pre-soak and washing if needed.
Treat white, bleachable fabrics with chlorine bleach instead of oxygen bleach. Algae are similar to mold and mildew in growth; chlorine bleach has long been used to remove mold, mildew and algae stains from outdoor surfaces and works by killing the organisms producing the stain. Dilute bleach per package directions for stain removal before using on fabric.
Treat outdoor vinyl fabrics like fabrics, not plastics. This means using noncorrosive cleaners like Simple Green or dilute chlorine bleach. Pool covers and other vinyls should not be treated with the chemicals intended to kill algae in the pool, warns a pool-chemicals retailer (see references).
Try an old-fashioned method to remove algae stains. Dip fabric in lemon juice (bottled or fresh), sprinkle stained areas with salt and hang the item outside on a sunny day. When it dries, refresh the bleaching action of the lemon juice and salt by sprinkling the item with water and hang it out again. Very stubborn stains may take more than one day of sunshine to disappear.