[kien]

what is the Eheim canister for?

As for the Herbie, it appears you have it set up properly in your drawing. The gate value allows you to adjust the flow of water from the Display Tank/Overflow box, into the sump. This effectively allows you to control the height of the water in the overflow box. Typically you want the water in your overflow box to be well above the main drain pipe so that you don't get that slurping and gurgle noise due to water and air getting sucked into the main drain pipe. The emergency pipe is left wide open in the event that something gets into your (slightly closed) main drain pipe and clogs it.