The common carp (Cyprinus carpio)

[Tartar Horde]

The "upper" pond SD was generally fed by, and on the same level as a nearby river etc, sometimes connected by a "cutting" from the river to the pond. The lower pond only has to be slightly less elevated than the upper for gravity to do it's work.

---

I suppose it's a matter of scale, you could set up a system that is small enough to be filled by Pipe etc, and still have enough room for a good head of fish.