Ten thousand goldfish raised by a farmer in Songjiang District died yesterday morning after he replaced the water in his pond with water from the nearby Dianpu River.
The river is a tributary of the Huangpu River and has been severely polluted for years.
Several other fish farmers in the area also complained about dead fish after changing water in their pools.
A local environmental protection monitoring team is investigating the situation.
Fei Minhua has been raising goldfish in water from the river for four years and says he has never suffered such a big loss. He alleges the problem was caused by a dyehouse in the upper reaches of the river, which he says is discharging toxic waste into the waterway.
"I noticed the water these days is a bit whitish but I didn't pay attention as dead fish caused by dirty water happened a couple of times before," said Fu. This time he hopes to seek help from local authorities and claim compensation from the polluter.
Authorities said it will be hard to blame water pollution on a single factory.
"The section of the river in our district is dead now," said the official with the district's environmental protection bureau surnamed Yin.
He explained that both ends of the river in the region are locked by water gates in the summer to prevent flood surges. That means the water is especially bad this time of year.
In addition, industrial waste and residential sewage have been discharged  into the river for years due to the shortage of sewage disposal facilities.
"We can hardly help him unless he points out which factory is suspect," added Yin.
Authorities warned locals should not use the river water for farming, otherwise they have to take the responsibility themselves.