Algae is beneficial to an aquatic ecosystem; however, when levels get too high there can be problems. Some algae can release toxic compounds, but the most common source of fish kills related to algae is oxygen depletion. ... It is the extraction of oxygen for respiration in water at night that causes most fish kills.

Can green algae kill fish?

These algae blooms are dangerous!

Blue-green algae blooms that occur in freshwater lakes and ponds can be directly toxic to fish and wildlife. The blooms produce a toxin that can kill fish and even mammals if ingested in large amounts.

Does algae cause fish to die?

During the day, the algae produce oxygen through photosynthesis, but at night, when photosynthesis stops, they and other organisms keep respiring, using up oxygen. So on warm summer nights during algal blooms, the dissolved-oxygen concentration sometimes drops too low for the fish, and a die-off can occur.