Perth Sheriff Court imposed a fine of £5,000 on Scottish Water on Tuesday after the company was found guilty of knowingly permitting detergent to enter the River Almond on the outskirts of the city.
The court had heard how in 2005 the company had been informed of leaking chemicals by officers from the Scottish Environmental Protection Agency but had taken several hours to send any staff to address the issue.
Once staff did arrive they failed to sandbag the leak that day, despite repeated requests from SEPA.
The resulting leak killed thousands of fish including trout and eels.
SEPA officer Pauline Silverman (Cookson) said: “It is disappointing that Scottish Water failed to mitigate the pollution incident when it had an opportunity and obligation to do so,” said SEPA officer Pauline Silverman.
“Blocking the flap valve at the outfall would have stopped some of the detergent from reaching the river, however this was not done until the following day.”