Yes you need an overflow box , tank (sump) with baffles and a return pump. Size of flow should be 3 to 5 time your display tank volume.
( 30 gal X 3 = 90 gph ) this is slow flow ... ( 30 gal X 5 = 150 gph ) ... This should not be confused with rate of flow inside your display
tank, which should much higher.
No over flow. I almost had to get rid of my fish. THey where without there filter for a week because of a tank leak. My mom got me more sealant which is good. so after it cures in a few days I can reset the filter and skimmer and hopefully all will be well. I had them on ebay for a 1 day duration so they can go to a home with a filter. I think maybe the hermit crabs leg pokeked through it and caused it to leak. it looked like a pin-hole