A typical sump setup doesn't use a float switch. An overflow (think of it as a hole in the side of the tank at the water line) is used to return any water above the "hole" back to the sump. So a return pump (Look at a Jebao 4000 or so aquarium pump) pumps the water from the sump up to the main tank. As the water rises it goes over the over flow and falls (through tubing) back to the sump.
There are some good youtube vids that will give you a good visual of how an overflow and sump work... no float switches involved.
There are several reasons you don't want to use a "sump pump" with a float switch... they pump WAY too much water and they will put so much heat into the water that the tank temperature will go up until the fish boil.