Identifying Black Algae in a swimming pool:
1. Black or Blue-Green spots with raised heads, not Free-Floating.
2. Harbors in rough areas of the pool plaster.
3. Does not Brush off the wall easily.
4. Found in pools even with proper filtration and sanitation.
5. Not to be confused with mineral staining, which can discolor the surface (black), but won't scrape off.
Where does Black Algae come from?
Black algae is very similar to dark spots found in the shower or in the caulking around the tub. It can arrive in your pool from a swimming suit that was used in the ocean, or it can blow in on a breeze, in the form of air-borne spores.
How to Remove Black Algae from a Swimming Pool
1. Balance the pool water - pH, Calcium Hardness and Total Alkalinity levels. Proper water balance is important in fighting all strains of pool algae, which prefers high pH and Alkalinity, and low Calcium levels.
2. Scrape off the heads of the black algae using pumice stone, trichlor tablet or putty knife. (note - only use these techniques on a plaster pool, these tips will damage other pool finishes). Using a quality steel bristled pool brush, go over all areas again, with vigorous brushing. Vacuum the pool very thoroughly. If you can vacuum to waste, this is preferred, to keep the black algae spores out of the filter.
3. Immediately after brushing and vacuuming, use a kill dosage of Silver Algaecide. For me, Silver Colloidal is my first choice, however not the most readily available. Another choice can be a Quaternary Ammonium compound, commonly called a "Quat". This is usually labeled as either Algaecide 10 or Algaecide 50. Use the 50, it's 5x as strong. Add an extra strong dosage, which will likely cause the pool a small amount of foaming. The foam will be broken down in the following days by a shock treatment.
4. While allowing the algaecide to work for 2-3 days, it's time to dig into the pool filter. If you have had a bad case of black algae, it is likely hiding inside your pool filter. For DE pool filters, remove the grid assembly and hose thoroughly. Hose the inside of the filter tank also. Soak the grid assembly in a 5-1 solution of water - bleach, in a large (clean) trash or tub. Allow the grids to soak for 5 minutes, then hose again to rinse. Then dunk the assembly in the pool for further rinsing. If you have a sand filter or a cartridge filter, the best advise is to change the filter media. Replace the sand or the pleated filter cartridge with new. In cases of heavy black algae, also chlorine soak the pool cleaner bag, hose floats, skimmer baskets, pump basket. Inspect behind the pool light and under ladder treads. Look in the tile line and in the corners of steps.
5. Shock the pool heavily with granular chlorine (Calcium Hypochlorite). Triple the normal dose, which usually means add 3lbs per 10,000 gallons of pool water. If in doubt, add more shock, especially in the areas where the bloom occurs. Sprinkle the shock directly over the area, or onto your pool brush that is positioned on the wall below the algae bloom. Scrub it in to the wall, and really dig your stainless steel bristled brush into any nooks and crannies to work on the roots of the algae.
In our little pool in California, we never did get rid of our Black Algae problem completely - until we employed the last ditch effort, replastering the pool. Buried it alive. Not sure if it ever returned, I moved on to college soon after and my parents moved to a smaller place (without a pool). I hope you never have to get to this stage. Black algae can be controlled but it takes vigilance, and a little humor won't hurt!