United Utilities has been fined £24,000, with costs of £4,554, for polluting the river Tame and killing hundreds of fish in June 2009.
The firm pleaded guilty at Tameside magistrates on 16 February to breaching regulation 12 (1)(b) and regulation 38(1)(a) of the Environmental Permitting (England and Wales) Regulations 2010.
The court heard that Environment Agency officers were alerted by members of the public to dead fish in the river in Mossley, near Manchester.
On visiting the Buckton Castle water treatment works, officers were told by United that sodium hypochlorite (bleach) had overflowed from a container.
The company pumped the bleach into a surface water drain, thinking it led to the foul sewer. But the bleach discharged into the river, polluting 1.5 kilometres of the watercourse and killing brown trout, chub and pike.
In a statement to ENDS, United said it regretted the incident. It has since updated its alarm system and modified the site drainage to stop the problem recurring.