You want to dissassociate the whats the filters rated for and what size tank you will actually be puting it on. Focus on gallons per hour (GPH) of the filter. Like others have said, shoot for 10x turnover rate of the entire tank per hour.
A standard filter for a 20g tank is probably 40-50 gph.
That means your only getting 2.5-3x turnover rate in the tank per hour; not sufficiant.
So, you would want to look for a filter thats was around 200gph to achieve, atleast, what is generally accepted on this site.