The number of dead fish in a stream has risen to 4,400.
The gudgeon, chub, dace, roach and other species have been found by the Environment Agency (EA) in Moor Ditch north of the Ladygrove estate since Saturday.
The EA said water samples indicate the presence of a detergent or cleaning product but it is still investigating the source.
A spokesman said there had been a “dramatic” improvement in the water quality since it was first called in.
He said: “Efforts continue to trace the source of the pollutant but we believe the discharge has stopped. Water quality is now returning to normal and live fish have been seen at various points in the Moor Ditch.”
Anyone with information about the incident should call the EA on 0800 807 060.
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