Siccy Synra 600 or 1000 skimmer pump. Vertex skimmers use lots of air. You might be able to find a small DC skimmer pump, problem being the pump exit must be real low, the Sicce pumps can be laid on their side, and most DC pumps are kinda of too big.
Going back to this thread, I remember that I was able to get a Tunze 9420 skimmer pump to work on my Vertex 150. The 9420 is just like the Synra line of Sicce pumps.