Fire crews have been trying to stop detergent foam flowing further down a river which was caused when a warehouse fire in Newton Aycliffe was extinguished.
The Environment Agency said about 100 small fish had been seen dead in the river nearby.
The fire started at the Stiller warehouse, which contained aerosol cans, on Friday.
Investigations into how the fire was caused are still continuing.
The warehousing and distribution site was evacuated on Friday, but no-one was injured.
Sandbags have been used to stop debris from the site falling into the River Skerne.
Phil Marshall, from the Environment Agency, said the foam had been caused by a reaction between fire water and factory products, mainly shampoo.
He said the foam had travelled down the River Skerne to the centre of Darlington.
"The fire service, in consultation with ourselves, has started to contain the water on site and recirculate it," he said.
"The products that have gone into the river so far will work their way down the Skerne and will become diluted as they go through the river system."
On Monday evening, the Environment Agency said it had seen the dead fish, such as minnows, in the river between Newton Aycliffe and Darlington.
At the height of the fire, eight fire engines were there and the smoke could be seen for miles around.
The area around the factory was cordoned off over fears about exploding aerosol cans.