Eutrophication of water bodies leading to the death of fishes is mainly due to the non-availability of

Eutrophication is the natural aging of a lake by nutrient enrichment of its water. 

Effluents from industries and residences can hasten the aging process dramatically. Lakes in various regions of the world have been badly eutrophied by sewage, agricultural, and industrial pollutants over the last century. 

Nitrates and phosphates, which act as plant nutrition, are the main pollutants. They encourage the growth of algae, resulting in unpleasant scum and foul odors, as well as depleting the water of dissolved oxygen. 

Other pollutants streaming into a lake, on the other hand, may poison the entire population of fish, whose decomposing lowers further the water's dissolved oxygen content.