Hundreds of dead fish were found floating in the river at Bishan-Ang Mo Kio Park by residents on Tuesday, causing a stench in the area that lingered till Wednesday afternoon.
About 400 fish were caught in clumps along the banks or on rocks by the three-kilometre long river, especially on the stretch close to Bishan Road. Many were decomposing under the hot sun.
National water agency PUB said that it was notified about the dead fish on Tuesday morning, and immediately sent contractors to clean up the river. The clean-up was completed by 3pm on Wednesday, the spokesman added.
PUB explained that lower levels of dissolved oxygen in the water caused by recent hot and dry weather could have killed the fish. It said that only two species of fish - tilapia and mayan cichlids - were found dead.
It assured the public that the water remains safe for activites, and that it would continue to monitor the situation closely. The spokesman added that raw water in Singapore's reservoirs is treated to meet World Health Organisation drinking water quality guidelines before being supplied to households.
Many long-time residents said this was the first time they had ever witnessed mass fish deaths in Bishan-Ang Mo Kio Park.
The picturesque river is part of the Kallang River, with the latter running from Lower Peirce Reservoir down south to Kallang Basin.
It opened in March 2012 after having been converted from what was previously a concrete canal, under a PUB programme to turn drains, canals and reservoirs into streams and lakes.