In swimming pools the ideal environment can exist where there are periods of zero chlorine. The problem for pool owners is the short life cycle of algae, sometimes as low as twenty minutes. Under normal growth algal blooms can take less than a day to mature into a green pool.
 

At the first sign of adversity the algae population go into a reproduction phase where two cells meet, parley and combine to produce eggs or SPORES. The size of the spores is less than 0.2 microns. DE Filters, filter 5 microns and above and sand filters 20 microns and above.
 

Algae will die from chlorine with concentrations as low as 0.05ppm but spores can resist chlorine levels of up to 10 ppm. Salt chlorinators do not achieve those conditions and a manual chlorine dose would need about 1-2 Kg of calcium hypochlorite equivalent to be effective.
 

Spores, however, cannot tolerate copper salts as copper attaches to the shell or (endospore) preventing germination. Quaternary algaecides are synergistic with copper salts hence the myriad of formulations for algae control.