Lincoln Magistrates heard yesterday that 360 fish died in April 2011 because a foul sewer blockage was left untreated for more than a week.
The Environment Agency, prosecuting, said sewage and trade effluent had spilled into a stream running through Heapham Lakes, Gainsborough, and continued into the River Till.
Environment Agency officer Sarah Gillanders said after the hearing: “An extensive clean-up operation was needed after this pollution and fish had to be rescued from the lake and moved to safety.
“Had Severn Trent responded sooner, the serious effect of the pollution could have been minimized.”
The company said the delay in tackling the blockage was due to problems with a new IT system. A service delivery manager said the company had spent £84,000 on investigating the incident and remediation.
Severn Trent previously polluted Heapham lakes twice in 2004 and once in 2007, with one incident resulting in 7,000 fish deaths.