Capeldrae, the tube is acually the outlet, it's connected to the pump. (This person has the pump in the back, but I will have it in the front.) The intake is not clearly visible in that picture, but here's one from a person who had his tank made by the same builder and it shows the grate of the intake. It's on the left side.

Source: Dutch Cichlid Forums
In this one the pump would go in the far right compartment and simply pull the water through the filter mats, which will have different densities.
I've used both these pictures to explain to my builder how I wanted the filter.