The tank will have at least one hole drilled in the bottom that should be outfitted with a bulkhead fitting. Typically this will be behind kind of a dam which water flows over. In the hole will be a pipe that water actually flows into to keep it from making an incredibly loud gurgling sound. Below the tank would be plumbed to a sump.
Most of the time there's a second hole which would be for the return. It would have (again generally) a smaller diameter pipe that goes up and over the dam.
This genes rally makes for a clean looking installation which could be right against the wall or in the wall.