Sewage flooding a river for hours was blamed on Anglian Water's slow response to the incident.
The company has been fined £18,000 for failures at a pumping station near Peterborough that an investigation said showed up their "reactive attitude" to pollution.
District judge Ken Sheraton said Anglian Water should have given "a higher category of response" to a second sewage pump failing because the site at Yaxley was operating at a lower capacity with another pump already out of order.
The pumps were meant to regulate the flow of effluent but had stopped working a week apart, meaning raw sewage poured into Pig Water Drain for several hours and the damage was done before an engineer arrived at the pumping station in Windsor Road.
Investigators from the Environment Agency said at least 60 fish died with no chance of surviving the incident in February 2019. The final death toll was believed to be higher, with the fish killed by reduced oxygen and toxic ammonia in the water.
A dog-walker reported a creamy substance on the surface of the watercourse. The woman said she saw fish leaping from the water like they'd been electrocuted or attacked.
Louis de Quincey, who led the Environment Agency investigation, said:
With only a single pump in operation, this was always likely to increase the chances of an incident. Anglian Water should have sped up its response time as a result. Many hours passed before a technician stopped sewage entering the water.
Anglian Water could and should have acted quicker.
An Environment Agency officer called to the scene saw effluent gushing into the "grey and murky" water from a pipe traced back to the pumping station, supposed to control the flow of sewage through the sewer network.
Anglian Water staff seemed blind to the ongoing incident - one employee telling the technician there was nothing wrong, that no sewage was being pumped into the river.
Roach, pike and eel were among 60 dead fish recovered from the scene, but fisheries specialists from the Environment Agency believe many more would have been killed by a sharp decline in water quality caused by the release of the effluent.
In polluting Pig Water Drain, Anglian Water pleaded guilty to breaching regulations 12 (1) (b) and 38 (1) (a) of the Environmental Permitting (England and Wales) Regulations 2016.
Sitting at Peterborough magistrates' court on 4 May 2022, district judge Sheraton also ordered Anglian Water to pay £10,957.80 in costs to the Environment Agency and a victim surcharge of £170.
