what would happen to an aquatic ecosystem when the water is polluted by insecticide and herbicide?

When insecticide or fertilizer enters the water ecosystem it either kill the algae or support it to grow . If the algae in water grows obviously it wil die . After its death the bacteria or any microbe tries to decompose it then it will consume most of the dissolved oxygen in the water which leads to increase in the BOD(biological oxygen demand) . In the absence of oxygen the fishes and other animals will suffer a lot and finally die. This again leads to the BOD increase and the dead bodies of animals will a fungal bloom which very undesirable . This is how the ecosystem gets disturbed .