Skimmers?

Hi everybody, researching equipment for the pond I'm building. The guy
at the pond store says I need a skimmer. Is that true? Been reading
about the veggie filter online and think thats the route I'm going. Can
I make a veggie filter out of a Bio falls waterfall? One thing I've
learned through many years of aquarium keeping, the people at the stores
are there for the sales. The groups I frequent for advice however speek
from experience. Once again, any and all advice is appreciated. Thanks