Hundreds of dead fish have been removed from Pandan Reservoir over the past few days by the authorities, and the clean-up was still ongoing yesterday afternoon.
National water agency PUB told The Straits Times that it had found low levels of dissolved oxygen in the reservoir water, which likely led to the deaths.
"This could be caused by a combination of factors such as dry weather, algae growth and decomposing organic matter taking up the dissolved oxygen," said a spokesman.
Tests on water samples showed the reservoir's water quality was not affected, but PUB said it would monitor the situation.
It added that it has a comprehensive system to keep an eye on the quality of the raw water in the reservoirs, and the water is also treated to World Health Organisation drinking water guidelines before being supplied to households.
Meanwhile, PUB has stepped up measures to increase the levels of dissolved oxygen in the water.
The reservoir usually has an aerator that runs around the clock and a fountain turned on thrice a day to stir the water.
They help to dissolve atmospheric oxygen into the water.
The fountain has been operating non-stop since Friday after the deaths of the fish.
"We have also deployed five additional portable aerators to the reservoir," said the spokesman.
She added that the rain yesterday morning has helped to increase levels of dissolved oxygen. "Rainwater creates movement in the reservoir water, thereby improving aeration and distributing the dissolved oxygen more evenly."
Dead fish were also spotted in Sungei Buloh Wetland Reserve last week, and some people claimed parts of Sungei Buloh Besar turned black.
The Agri-Food and Veterinary Authority said it is working with the National Parks Board to investigate the deaths.
It warned that it would take action against farms found to be illegally dumping waste in the water.