Return pump goes into your sump and pumps the water from below in the sump to your tank. BUT you would have to have a way of getting the water from your tank down below into your sump. You could drill the tank OR get a hangon back overflow.
The sump is useful to put all your equipment like heaters and it will increase the amount of water you have in your system which helps make it easier to take care. More water volume the better...
180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18