For a 20g you have reasonable choices among internal, HOB and external cannister filters. Fluval internals are recommended my many members here. AquaClear HOBs are recommended as flexible by many here. External cannisters, which are relatively more expensive than the previous two types but have most of the advantages are debated at length on the forum. The many features and trade-offs between the four main contenders, Eheim, Rena, TetraTec and Fluval are extensive and so are the members various biases.
Media volume, flexibility of media choice, ease of media cleaning, noise, ease of seal maintenance, ease of impeller maintenance, ease of priming, reliability of pump, strengh of pump, flow rate, quality of plastics, quality of intake/ouput attachments, ease of service, ease of replacement part acquisition, quality of manuals, quality of web site, are among the attributes aquarists end up determining or having opinions about with regard to filters.
~~waterdrop~~