Anglian Water has been fined £20,000 with £28,837 in costs for polluting a river in Essex where hundreds of fish, eels and shrimp were killed.
Sewage leaked in to the River Crouch at Wickford after a pumping station breakdown, Basildon Crown Court heard.
The firm admitted causing poisonous, noxious or polluting matter to enter controlled waters on 17 August 2009.
Among the 666 fish killed were 187 eels, a protected species in decline in East Anglia, the court was told.
Others killed included large perch, chub and flounder, bullheads and stone loach. Several thousand dead young fish were also seen but were too small and numerous to count.
Mark Watson, prosecuting, said the pollution affected at least 2km of the River Crouch which flows through protected environments with local, national and international conservation status.
Anglian Water believed a transient fault with the electricity supply caused the pumps to fail at Long Meadow Drive Sewage Pumping Station, he said. But there was no evidence as to why the system failed on this occasion.
In a statement Anglian Water said: "Our investigations showed our pumping station was operating as it normally should, until an unpredictable and temporary fluctuation in the electricity supply.
"This was despite all precautions we had in place to try and prevent such an occurrence. This incident was entirely unintentional and we are sorry that it happened at all."