The fish died after living in the aquarium for many years

In an aquarium the water quantity is limited and fish excrete ammonia through their gills and body, this dissolves in water and creates toxins.

Explanation:

In an aquarium the water quantity is limited and fish excrete ammonia through their gills and body, this dissolves in water and creates toxins. Over some time some bacteria develop in water which converts this ammonia into nitrites and then into nitrates. Till this process is complete, the aquarium remains a death trap.

There are some reasons for a fish to die early,

This one is common for beginners. They don't have any idea about the nitrogen cycle. And they simply buy a fish tank and fish on the same day, go home and set it up. Ammonia spikes - Even in a cycled tank, you can have occasional ammonia spikes due to overfeeding. Incompatible tank mates - If you put a too docile fish with a very aggressive cichlid, it will get harassed and eventually die due to stress.