What type of algae is considered hardest to kill?

Black algae is considered the hardest type of algae to kill primarily because of its unique structure and resistance to treatment. It forms a dense, protective layer on surfaces, often associated with porous materials like plaster in swimming pools. The algae have root-like structures that anchor it firmly to the surface, making it difficult for chemicals, such as chlorine, to penetrate and effectively eliminate it.

Additionally, black algae can often withstand a range of chemical treatments as it tends to thrive in environments that are filtered poorly or where sanitation levels are not adequately maintained. Its ability to adapt to various conditions allows it to resist typical algaecides, necessitating more aggressive treatment methods, such as scrubbing with a wire brush and using specialized algaecides.

In contrast, other types of algae, such as green or yellow algae, are generally more susceptible to standard pool chemicals and can be eliminated with routine chlorine treatment or algaecide application. Blue-green algae (also known as cyanobacteria) can be troublesome but still does not exhibit the same level of resilience as black algae. Thus, the difficulty in controlling and eradicating black algae confirms its status as the hardest to eliminate in pool environments.