You were very likely seeing a flow through system, filtered through a sump, with automatic top offs fed from a fresh water supply.

But for a single pond, which is in essence, no more than a single very large tank, you don't need anything so elaborate.

For good pond filtration info., ask the folks over on the Pond Board. Lots of experienced and helpful ponders there.

You could also set up a sump system for your pond. It can be merely a DIY job and hidden under some landscapeing for camoflauge. Pet stores usually have UV filtration that kills bacteria, parasites and free floating algaes. There is alot of indoor aquarium equiptment that can be used in a pond, or can be replicated on a larger scale for pond use.

Depending on the size of your pond the newer filtration systems usually consits of bottom drains hooked to a settlement chamber/filter with a return pump. A surface skimmer is usually part of the system. As BW said the pond forum has many experts on the latest systems. A store multi tank setup wouldn't work well on a pond [no bottom drains to collect gunk and leaves].

Well there you go. Good point James on the leaves and gunk. I kind of thought of a way to use a sump system for a pond, whenever I build one at the new house (whenever thats done). I will incorporate a bottom drain into the system. Thanks for the food for thought.

woeisme, I have a 180 gal outdoor fiberglass pond with just an internal filter and I dump it every spring to clean it out. When I get a bigger pond i'll probably have 2 bottom drains and a skimmer. I'm sick of doing maintenance.