Cause of death fishes in Serangoon River undetermined

Can this be happening? With advance test equipment & technology, The National Environment Agency (NEA) and the Public Utilities Board (PUB) are unable to determine the cause of death of the fish that were found in Serangoon River earlier this month.

On 16 May, a large number of dead fish, mainly tilapia, were found floating in the river. Tests carried out by the NEA found no toxic substances in the water. The two agencies said the cause of death could be a lack of oxygen in that section of the river. Current tests confirm that the water quality in the river has returned to normal.

Sudden lack of oxygen? Did someone unintentionally, accidently dropped a piece of Dry Ice into it?