Environmental groups have said they are worried about the long-term effects a cyanide leak into the River Trent in Staffordshire will have on wildlife.
The Environment Agency said the cyanide levels in the Trent between Stoke-on-Trent and Yoxall had "greatly reduced".
However, tens of thousands of fish died and the Staffordshire Wildlife Trust said it feared there could be long-term effects on animals and plants.
Anglers said the pollution leak had been a "complete disaster".
The river had only been restocked with 200,000 young salmon last year.
The Environment Agency is still advising people and animals to keep out of the 30-mile stretch of water as a precaution.
It previously said it would prosecute those who polluted the river under the Water Resources Act.
Sue Lawley, of the Staffordshire Wildlife Trust, based at Wolseley Bridge, near Rugeley, said she was "very worried" about the situation.
She said any dead fish had to be removed quickly to prevent them being eaten by birds or mammals.
She added the trust was concerned that although otters do not usually eat dead fish they find, they could eat fish that had been contaminated but had not died.
Ms Lawley said they were also particularly worried about kingfishers and herons, along with crayfish that are native to the Trent.
She added that it was possible for cyanide to persist in the environment for a long time as it can be absorbed into plant and animal tissue.
The Environment Agency has said it could not give more details of where the cyanide came from for legal reasons.
Officers began pumping oxygen into water on Tuesday night to reduce the pollution levels.
The cyanide led to partially-treated sewage being released from Severn Trent's Strongford treatment works in Barlaston.
Drinking water has not been affected.
The Environment Agency said it had not received any reports of humans or pets being affected by the contaminated water.