As an aquarist I'd say the likely cause IS probably lack of oxygen, but compounded by overcrowding, and pollution of the water by chemicals and organic matter.
Have you seen the state of the average body of water in the Phils ?
The situation was bad enough in THIS country for long enough.
There it is second nature to toss anything and everything either to the side of the road or into the nearest water course
Surely the govt. dept. concerned has done some testing ?
