I guess I never really looked at what a real sump filter is
A sump would be like a 2nd tank ususally kept out of site. Water is skimmed off the display tank and into the sump. The sumps sole purpose is to biologically recondition the water. In SW sumps generall contain a deep (verry deep like 6+ inches) of live sand for a large anoxic filter, as well in some cases as more traditional biofilter media, or plants.
Water is taken from the main tank, and is run throug the sump which is optimized for bio-filtering, then back up to the display tank.
This is a whole nother subject which can take months to read into and understand completely.
In general sumps are used to filter much larger volumes of water than a 20 gallon tank.