Appropriate filters for goldfish pond?

So I'm considering asking my grandpa to stock his tiered waterfall pond with shubunkin goldfish, but I know they have a wicked nasty bioload and thus need a powerful filter. I'm hoping to put at least one filter in the pond per section stocked with fish. Should the filter be powered exactly for that volume of water, or should it be more powerful to account for the goldfish bioload? The water circulates down each tier and then up to the top one through a pump, but I'm assuming one filter in one tier won't be enough despite that. Sorry if this is a little confusing, here's a picture in case you need a visual. I know @Deanasue keeps goldfish, so maybe you know how to pick filters for them? Thank you so much

I have a pond filter that I bought from Lowe’s. It’s good for up to 800 gallons although mine is only 150G. I’ll try to find a pic. This the one I have. Are you in the U.S.?
Lowe's › ... › Pond Accessories
smartpond Black Pond Filter in the Pond Accessories department at Lowes.com - Lowe's

Improves water quality and clarity for ponds up to 850 gallons. Can be easily hidden in or outside the pond. Includes 12 bio-balls and 2 filter pads that together ...