Things You'll Need
Swimming pool shock (calcium hypochlorite)
Chlorine test kit
Pool brush or other soft-bristled brush
Pool vacuum (optional)
Additional pool filter cartridges
There are three main types of algae, all classified by color: green (the most common), mustard (yellow) algae, and black algae -- the most dreaded as it can never be totally killed, only rendered invisible and controlled. Pink algae is another pool invader; not really an algae but a fungus that creates slimy colonies colored either pink, white or gray. If you notice what appears to be "gray" algae in your pool, it is most likely either pink algae, or dead algae. Dead algae turns gray as it dies and must be removed from the water through filtration. Regardless of the source, however, a few steps will kill any live algae and remove the dead.
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Add enough shock (calcium hypochlorite) to raise your chlorine levels to at least 10 ppm free chlorine (which is 10 times the recommended everyday chlorine level of about 2 ppm). Follow the product instructions when using shock, calculating the amount required according to directions. You can also use a chlorine test kit to check chlorine levels.
Turn on your filtration system, if it is not already on. Water must be circulated around the clock during the entire algae treatment and removal of old, dead gray algae. This will aid in the removal of algae bodies killed by the chlorine, as well as good circulation of chemicals.
Remove any items in your pool that may contain algae on the surface (balls, rafts, ladders, and other accessories). Wipe them down with chlorinated bleach mixed with water (at about a 1:1 ratio). Allow them to dry thoroughly and wait until the pool is clean before reintroducing.
Brush the side walls and floor, using a pool brush or other soft-bristled brush. Sweep towards the pool drain or filter, if possible. Make sure to work into any corners or crevices, where algae may hide. Follow with a pool vacuum if you have one -- although this is not essential, just helpful.
Observe your swimming pool while the chemicals work. Watch for clearing of the water, which may take overnight in heavy or persistent algae problems. Brush periodically while you wait for the problem to clear, and add more shock if the level falls below 5 ppm while you treat your pool.
Backwash your filter, or change the cartridge, if the need arises -- pressure will raise about 10 psi when a filter needs to be back-washed; other pool filtration systems require the filter to be changed as the suction and output into the pool decreases; hold your hand over the inlet and outlet hoses in the pool to check.
A commercial algaecide may be added during treatment, after the super-chlorination (shock) treatment. However, chlorine alone is all that is needed to kill algae.
Carefully follow any product instructions; pool chemicals are not safe for ingestion or use by children.