Fluval filters are really good, but I would prefer something a little less powerful on a 20 galon. Go with the fluval internal filters or another aquaclear. What part of your aquaclear is going? Interestingly enough, they all use the same motor until you get to the 500, so if it's just an impeller problem, all you need to do is replace that. If you need to replace the motor, you might as well but a whole new filter (replacement motors cost around $20). I'd suggest the aquaclear 200 for that sized tank.
What kind of smell exactly? Different smells are the symptoms of different problems. As a band aid solution, add carbon to the filter. That will get rid of the smell, at least somewhat, but not necessarily fix the underlying problem.
(Message edited by brian on March 24, 2005)