The discharge occurred in November 2001 following the failure of a corroded valve on a slurry store at Carden Livestock Company's premises. Effluent poured out of the tank, overwhelming a safety valve which was itself in poor condition.
The slurry polluted 7.5 kilometres of the Gadie Burn and river Urie below the farm, killing between 3,000 and 5,000 fish - mainly young trout and salmon. Analyses by the Scottish Environment Protection Agency revealed the slurry to be 90 times more polluting than crude sewage.
The incident also disrupted traffic after a road junction was submerged in slurry.
Appearing before Aberdeen sheriff court on 19 December, Carden pleaded guilty to causing effluent to be discharged to controlled waters, contrary to section 30F of the Control of Pollution Act 1974.
The conviction was the company's second in two years. It was fined £500 in 2001 over a pollution incident caused by 5,000 litres of pig slurry.
After the hearing, a SEPA spokesman commented: "This was a very serious incident caused by poor maintenance of a slurry store. It had devastating consequences for the river, but it was preventable."