The external version is the Aquamedic Turboflotor Multi SL. It can be used in sump, or HoB
either on tank or on sump.
Here's the documentation for your skimmer:
One of the main reasons for a skimmer being fed from the drain (gravity fed) is that the water is taken from the tank's top 2mm or so where the water is most protein rich.
In some gravity fed skimmers, no pump at all is needed.
Just google "Gravity fed Skimmer Benefits" and the like, and you should find all your answers.