Black Algae

Perhaps the most difficult to remove and misunderstood type of algae. The difficulty in removal is due to the strong so-called roots or protective protein layers over top of this so-called black algae.

• * What most pool service technicians and pool owners call black algae is most likely cyanobacteria. We will refer to the cyanobacteria in this article as black algae. (Cyanobacteria also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. In this article Cyanobacteria is referred to as black algae in the pool industry, since it is the common name referred to and acts very similar to algae that also uses photosynthesis.)

Black algae will appear as dark black or blue/green spots, usually the size of a pencil eraser tip. The roots can extend into the plaster or tile grout imperfections, and unless the complete organism / roots are destroyed, a new head will grow back in the same place. The heads also contain protective layers to keep cell destroying chemicals from entering the organism. Like yellow algae, black algae strains can bloom even in the presence of normal sanitizing levels of chlorine in the water. This problem is made worse when the surface of the pool has little chips, pops or cracks that allow the algae to grow underneath the plaster and keeps its roots out of the way of the algaecides and Chemicals. Expect to always have this problem re-appear if you have rough, chipped or cracked pool surface.

If draining your pool is an option, (ok if daily outside air temperature is under 80 degrees) you may consider draining your pool and doing a CHLORINE WASH and scraping off the black algae, and sanding any rough “algae adhering” spot smooth.

If draining is not an option, we have found this INTENSE CHEMICAL TREATMENT to work the best on most pools. Turn off the pool circulation, and keep pool from running for 24 hours. Use an algae eraser to scrape and expose black algae surfaces. (The black will remain you are only disturbing the protective layer.) For all the black spots on the bottom of the pool, broadcast tri-chlor granular chlorine over the entire affected area. Spots on the side of the walls will need special attention. Next add 2 gallons of liquid chlorine for each 10Lbs of tri-chlor used, pour it slowly in a column allowing the heavier liquid to sink to the affected areas. Wait at least 24 hours.

Minimal Treatment: The next day, use the algae eraser again on the affected areas especially on the walls. (The algae on the bottom may be gone, but if black or gray is still present scrap with algae eraser) Turn the pool back on to regular filtration hours and add the Silver algaecide (one quart per 20,000 gallon pool) in small amounts slowly all around the pool with the pool circulating. Finally add one gallon of liquid chlorine for every 10,000 gallons of pool water. Yellowing or slight discoloration of the pool surface is possible where the tri-chlor granular treatment is applied.

You may use the algae eraser from time to time, and do the “minimal Treatment” again in the future as soon as the algae re-appear.