Whatever the reason, chances are that the fish was sick or
injured long before it died. One of the biggest problems I've found that
people have with their aquariums or ponds is they don't know what to look for or
they just don't look at all. Before you can diagnose a symptom you need to
know what a healthy fish looks like. Ideally you will know the habits and
mannerisms of the individual species. Then again ideally, I'll win the lottery and I can
stop sucking fish crap, moving huge rocks, and scraping pond scum for a living.
Then I can just do that for fun!
Healthy fish are active and react to stimulus. They hold their fins erect and are generally swimming around looking for food sources. The eyes are clear and shiny. The gills and mouth move at an even rate processing oxygen (about one or two "gulps" per second). They float in the water parallel to the substrate. The body is generally sleek and streamlined. Their behavior is that of the species, be it schooling with others or defending their territory. They learn that people feed them. They recognize the individual person that feeds them. They react to your presence the same way, all the time, whether you feed them or not.
Fish are amazing creatures and didn't ask to be captured, bred, and then stuck in a tank or pond. If you are keeping aquatic life it is your responsibility to provide them with the best care you can give them. Coming to this website is a great start.