Protein skimmers are great for highly loaded Koi ponds with hig feed rates, or for those who want the ultimate in water quality. They work by flocculating out dissolved proteins against a mass of fine small bubbles flowed in contact with pond water. Hence they typically have two water pumps – one to circulate water from in the unit through a venturi to create the bubbles and the second (can be a side stream of your main pond pump) to feed water from the pond through the unit. Aim for 25% to 40% of pond volume per hour though the unit.
Protein Skimmer without venturi pump (required to deliver bubbles to skimmer, you can fit your own rated at 10 to 12kl/hr to drive the venturi).