FISH died in the River Colne after its waters turned green.
Dead fish were seen in the river near King's Bridge at Newsome on Tuesday.
They were thought to have been washed downstream from Milnsbridge, where a dye turned the Colne green on the same day.
However, an Environment Agency officer who checked the problem at Milnsbridge said the dye had now passed out of the watercourse and more fish deaths were unlikely.
An agency spokes- man said the number of fish killed was not serious.
He added: "We received a complaint from a member of the public about the Colne running green in Milnsbridge.
"This was followed by a second complaint, alerting us to a couple of dead fish spotted downstream.
"We believe the cause of the discolouration to have been some green dye released into the river.
"When our officer went to inspect the river she could find no dead fish, but spotted plenty of live ones."
The source of the dye is not known.