The life of a fish can be very short. Especially those purchased at chain pet stores and looked after by working moms who can barely remember to shave their legs, take something out of the freezer for dinner, or return DVDs ontime let alone clean and feed tiny fish tucked away under the clutter of elementary age wonderment and dirty socks.
I am sure there are fish out there who are able to withstand the elements of hurricanes and typhoons and tsunamis or the occasional over-feed and cloudy tank, but lately, the only ones I have come across tend to stroke out if the water gets too cold or someone forgets to feed them on a regular basis.
The good news for those fish is that whatever burst of emotion their passing can bring up for the owner fades within some small proportionate amount of time to the length of their life.
Except for the nagging guilt of the actual care-taker which tucks itself away underneath layers of algae and soggy fish pellets only to erupt forth at an unplanned, unannounced and definitely unceremonious time.