Re. the settling tank within the filter:
I love this idea. We've recently done a pond with a 2chamber flow-through filter. Chamber 1 is (was) just chunk home-made charcoal. Chamber 2 was is course, then finer mattress sponge. The sponge does not need maintenance... ever. However, the charcoal sank and filled up with crud, even with the course (thumb to fist size) charcoal, and we kept having to dial down the pump as gravity wasn't keeping up. SO- course, settlement, bacterial flock and copious beasties can do the trick and keep your water clear for 3months before you have to deal with it. Details: pond is overplanted 800gal with most of it big & shallow at 7" deep. Sump is 3'x2'x2, with the 3' partitioned into the two chambers (thus each is 18"x2x2). Stock is just fancy goldfish, african & chromide cichlids, rainbowfish, danios & guppies.
Back to my question, then: how about using a round chamber, rotational inflow for centrifugal forces to aid settlement? Has anybody used/tried?