When it comes to mechanical versus biological. I have to think Bio is more important as stuff breaks down it creates ammonia which kills fish.
Well Bio IS more important that mechanical. But, bio happens naturally and mechanical does not. If you go with a bottom up design with the mechanical filtration, alot of the large debris can be flushed out via a bottom drain before it completely decomposes and produces ammonia.
Mechanical just collects large particels but they still break down and create ammonia no matter where they are in the pond or collected nicely in the filter.
This is not true. Almost all the media used in these skippy designs not only perform mechanical filtration but do double duty and provide and excellent breeding ground for good bacteria. For instance Koiguys design using scrubbie pads.......