Black Algae: What It Is and How to Get Rid of It

How to get rid of black algae

It’s the bane of every pool custodian’s existence! Those pesky black spots that develop on the pool walls, usually in hard to reach places. They’re an eyesore! And, they seem almost impossible to remove.

This is the notorious black algae. Unlike other varieties of algae, it is tenacious and extremely tolerant of most pool chemicals. The natural habitat of black algae is actually found in natural bodies of water, both salt and fresh. It gets into your pool through cross-contamination. For instance, when somebody swims in the ocean or a lake, and then gets into your pool wearing the same swimsuit.

Black algae are not as susceptible to algaecides as other varieties are. Part of the reason for this is that it’s not really algae at all. The proper name for black algae is cyanobacteria. The bacteria grow in dense colonies, which root themselves deeply into your pool wall and form a protective coating that easily resists chlorine and algaecides.

It seems that opinions are divided on whether black algae is harmful. There are sicknesses associated with water infected by black algae, but it appears that they are not directly related to the cyanobacteria but rather to other microbes that make themselves at home in and around the cyanobacteria colonies. In public pools, it’s safer to keep swimmers out until you clear the black algae.

Getting rid of these pool invaders requires a combination of chemical treatment and good, old-fashioned elbow grease. The best thing to use is a hard-wire brush together with chlorine tablets. Scrub the algae colonies until they disappear from sight. But, remember that this doesn’t mean they’re gone; their roots are still in the wall. So, take the chlorine tablets and rub them into the spots. Then, you need to make sure that the pool no longer provides a favourable environment for the organism. Do this by shock treating it with three times the amount of chlorine you would normally use. Run the pump for 24 hours and then shock the pool again with about twice the amount of chlorine you would usually use. Follow this procedure until the spots are cleared and don’t come back. Then go about rebalancing the pH and chlorine content of your pool as normal.  

It might also be worth putting up notices asking swimmers to use only clean swimsuits and accessories, but this is difficult to enforce. From the pool keeper’s perspective, the best defence against black algae is the maintenance of the utmost sanitary conditions in the pool.

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