Sumps are widely used in Marine setup and the refuge , usually called 'refugium" is most commonly part of the setup, In fresh water you do not need one.
An extract from wikipedia
In fishkeeping, a refugium is an appendage to a marine, brackish, or freshwater fish tank that shares the same water supply. For some applications water flow is limited in order to protect plants or animals that require slow flow. The refugium light cycle can be operated opposite to the main tank, in order to keep total system pH more stable (due to the uptake of acid-forming CO2 by the refugium during its daylight hours). One size guideline is 1/10 main tank volume.
A refugium may be used for one or more purposes such as denitrification, nutrient export, plankton production, circulation, surface agitation, oxygenation or aesthetic purposes.