The $1 million plant has doubled the capacity to provide oxygen relief to the environment upstream of the Kent Street Weir.
Environment Minister Albert Jacob said the new plant would ensure an extra 2.2 kilometres of the river could have adequate oxygen levels at all times.
When oxygen levels are low, it can harm aquatic life. Summer and autumn is when levels can fall because of the decomposition of excess organic matter and nutrient.
Adequate oxygen levels help provide a refuge for fish, improve nutrient cycling, speed up the breakdown of organic matter and prevent offensive odours.
Mr Jacob said the Government had also invested $1.4 million to upgrade the two older oxygenation plants upstream of the Kent Street Weir.
They service 2.3km of the Canning River.