External sumps, also called refugiums, are secondary tanks that are attached to the main tank through a common water filtering system. The refugium/sump tank sits outside of the main tank and is useful for holding and hiding filtering equipment, raising food like krill and plankton, adding additional water flow to the tank and giving small creatures a "refuge." Because a sump includes your filter and a secondary tank, it's easy to set up using your existing equipment and an old, discarded fish tank.
Make sure you have adequate room for your sump aquarium. The secondary tank should sit below the main tank, to use gravity for water circulation if necessary. The sump pump tank may also be placed behind the main tank if it's going to be used to store and hide the filtering equipment.
Empty both tanks of water before you start. This will make the process cleaner and safer. Empty your existing pump and filter of water as well.
Drill two holes in the side or back of your main tank, toward the top of the tank (depending on where you intend to put the secondary tank). The diameter of these holes should be slightly larger than the diameter of the plastic hose you're using, so that the hose fits securely into the holes. Insert one piece of plastic hose into each hole and use the sealant to secure them. Make sure the hose is long enough to reach the secondary aquarium and the filter that is now held there.
Move your filter and pump to the secondary tank, where they'll now be housed. Set them up, but don't turn them on. Attach the hoses that run from the main tank to the system; one hose should be attached to the sucking function of the pump, while the other should be attached to the pumping function. This will keep water circulating between the two tanks, and filtered.
Fill both tanks with water and turn on the pump and filter. Monitor the system and adjust the rate on the pump to avoid overflowing either of the tanks.
Purchase plastic hose at fish supply shops. Always give new equipment an hour to settle before you reintroduce live animals.
Turning on the pump and filter before they are filled with water will burn them out.