There is mass mortality of fishes in a pond. What may be the reasons?

The following can be the reasons for the mass mortality of fishes in the pond.

(i)Thermal pollution

(ii) Addition of poisonous (mercury) compounds in water

(iii) Due to blockage of gills with any pollutant.

(iv) Domestic Sewage

(v) Eutophication

(vi) Thermal Waste Water Discharge