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However, excess levels of nitrates in water can create conditions that make it difficult for aquatic insects or fish to survive.

Algae and other plants use nitrates as a source of food.  If algae have an unlimited source of nitrates, their growth is unchecked.

Why are high nitrate or nitrogen concentrations in water a problem, and what can be done to maintain safe levels?

Excessive concentrations of nitrate in lakes and streams greater than about 5 milligrams per liter (measured as nitrogen), depending on the water body, can cause excessive growth of algae and other plants, leading to accelerated eutrophication or "aging" of lakes, and occasional loss of dissolved oxygen.

It is primarily caused by the leaching of phosphate or nitrate containing fertilisers into lakes or rivers. Some algae and blue-green bacteria thrive on the excess ions and a population explosion known as algal bloom occurs. This growth is unsustainable, however. Perhaps because another nutrient becomes limiting, massive death among the bacteria happens, and they float to the surface.

Oxygen is required by all respiring animals in the water and it is replenished by photosynthesis of green plants. The oxygen level is already low because of the population explosion and further oxygen is taken up by microorganisms which feed off the dead algae. The layer of dead algae also forms a surface layer through which little light can pass, meaning the rate of photosynthesis decreases and the oxygen level becomes depleted. Due to this, fish suffocate.