Common reason fish die in a fish tank and what does it have to do with nitrogen cycle?

Ammonia poisoning resulted from incomplete nitrogen cycle or no filter at all, shock from sudden change in water temperature and (or) water chemistry, overfeeding.

These are the leading causes for fish dying in a fish tank. In fact, the average life span of fish sold in the U.S. is only 3 weeks.

Nitrogen cycle is the most important of them all, since it is unknown for most new fish keepers.