For some reason, Red Sea doesn't list required pumps for their skimmers (probably an attempt to sell their own pumps). Assuming your Berlin skimmer is a Red Sea model, you can go to Marine Depot to see what is recommended. Here's what it says:
"Red Sea Berlin skimmer requires pump. Rio+ 2500 or Rio 3100 is recommended for in sump setup. An Iwaki 30RLT pump is recommend for external setup. Overall height is 21".
"Red Sea Berlin XL skimmer requires a Rio 3100 or Iwaki 40RLT . Overall height is 33".
"Red Sea Berlin HO skimmer requires Rio+ 2500 or Rio 3100 pump. Please allow 9" depth for proper installation.