Why does bacteria and sewage cause the death of fish?

This question can be broken down. Sewage contains organic matter which can be broken down, and bacteria need nutrients which they can get from digesting organic matter. Any form of digestion with aerobic bacteria will use up oxygen, as oxygen is used in respiration to provide energy for this digestion. From this, oxygen levels in the river will fall and so there is not enough oxygen for the fish, meaning they cannot respire and so die.