Severn Trent has been fined £15,000 for releasing sodium hypochlorite bleach into the river Churnet in December 2008.
The company appeared at Stoke-on-Trent crown court on 21 April to plead guilty to breaching section 85(1) of the Water Resources Act 1991. Costs were £3,493.
Environment Agency officers were alerted to the incident on 27 December 2008. They saw many dead fish and smelled chlorine along the river, which is part of the river Trent catchment.