The surface run-off of nitrate from agricultural fertiliser into freshwater ponds and lakes can cause the death of the fish. Explain how this happens.

This question crops up a lot and may be up to a five mark question, if you learn the key points and terms you can be confident of getting all the marks you can here! 

First of all, an algal bloom occurs which blocks light from penetrating into the water. Algae can grow rapidly if nitrate, a food source, is less of a limiting factor.

Because the light can't reach the underwater plants they die as they can't photosynthesise (meaning they can't make the sugars vital for life).

The saprobiotic microorganisms  have aerobic respiration, so they use up oxygen in the water. Because the plants in the water have died, the oxygen levels can't be maintained. 

This results in the death of the fish that the question asks about because there isn't enough oxygen in the water for them to respire.