[Global] Like all green plants, algae produce oxygen during the daylight hours as a by-product of photosynthesis.
This is usually a major source of oxygen in fish ponds. In darkness, however, all plants consume oxygen, including algae. Algae blooms in natural water bodies or fish ponds normally produce much more oxygen in the daylight than they consume during the night, but some situations reduce the amount of oxygen a bloom produces without reducing its nighttime oxygen consumption.
Trace minerals or nutrients needed by the algae are occasionally used up, causing some or even all, of the bloom to die back temporarily. The resulting bacterial decomposition and loss of normal oxygen production can lead to oxygen depletions and fish kills. Pond water generally changes from a deep green to black, gray, brown or clear after a phytoplankton die-off. An additional problem caused by dense blooms, especially in excessively deep ponds, is stratification.
View original article at: If algae produce oxygen in a pond, how can having too much algae cause an oxygen depletion?