Ok, so I know they are a place where you can put heaters, protein/phosphate skimmers (for saltwater) etc., and a place where you can do custom filtration, as well as using a normal store bought filter, as well as lessening the bioload by adding more water quantity. What I want to know is, how do you get the water from the main display tank into the sump, and back again? I know it probably uses some return pump, but I can't really find anything (like maybe a website titled "sump basics, for total idiots").