Thames Water has been fined £2m for a "reckless failure" that led to the polluting of the River Thames.
In 2017 the water company received a record £20.3m fine from the Environment Agency for pumping 1.9 billion litres of untreated sewage into the Thames.
The latest fine was imposed after raw sewage flowed into a brook near Milton-under-Wychwood in Oxfordshire in 2015.
About 150 bullhead fish died across a 50m stretch after sewage flowed into the water for at least 24 hours.
Before sentencing Thames Water had admitted to breaching environmental regulations.
Defending the water company, Kerim Fuad QC told Oxford Crown Court Thames Water had invested £40m in improving waste water sites.
He added the damage to the environment was "extremely regrettable but the cause of this unfortunate event in many respects remains unknown".
He said the company, which has an annual turnover of £2bn, operates more than 4,700 pumping stations.
But in sentencing Judge Peter Ross said the number of pumping stations was "irrelevant".
He said: "I am sure that more effective management, training and the establishment of a proper culture would have ensured this episode could not have happened."
The Environment Agency said more than 1,000 alarms had been ignored at the pumping station in the Cotswolds, which informed staff about faults with the removal of sewage.
Thames Water was also told to pay legal costs of nearly £80,000.
But Judge Ross said he would reduce the fine if the water company donated £200,000 to three local environmental charities.