typically goldfish are the most unknowingly neglected fish due to the fact that most believe all they need is a bowl. Gold fish are very dirty fish and naturally produce a lot of ammonia. They need a LOT of filtration this is why the gold fish in a bowl doesnt last too long. Tropical fish are MUCH easier in that the do not require as much space per fish as a gold fish does. This is a great site that lists the different species of fish and will even tell you what ones do best with which ones etc
http://www.elmersaquarium.com/000tropfishcareguides.htm VERY imformative even has gold fish info