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.

Is green algae bad for fish?

Green algae are considered as “good” algae, but their growth should be kept under control so they won't deprive the fishes of nutrients. Some aquarists let green algae thrive to serve as a dietary source for their fish.