Um.. I had the same problem, then i got a slightly bigger tank with a filter, and all you have to do is clean the filter and the tank wont smell as bad. Because gold fish DO smell lol
Gold fish eat just about anything and can give off a really offensive smell. Also, the water could be causing some of the problems if its dirty. Gold fish need a large space to live and if it is in a small tank then it will smell bad. Maybe see into getting some smaller fish. Gold fish are messy and dirty fish.