Mucky, You know I think this should have been brought up in the thread you have about Why you don't agree with Bogs. Or similar way you named it, I didn't go back and look. :zap: My point is, same principle and the two have very close similarities. There is a natural processes that shouldn't be ignored, but there is also the proven fact that if a pond is ignored and it has rocks and that bottom is disturbed (Koi digging, cleaning and maintaining, replanting, trying to net a fish) something is eventually going to disturb that bottom and your going to release that poisonous H2S and then in most cases, catastrophe.
There are so many variables, differences in style, and even goals at what you want out of a pond. I honestly believe I can set up a pond with rock bottom, and one without and be quite successful with both. If I approach each with mind set that each pond has it's own specific design and maintenance that needs to be monitored and maintained and not use the Basic principles of "most Fish keeping lined Ponds" Just as a Bog can be done, But there is always that knowing fact that there is going to be all that nasty mess being trapped in the bog gravel bed just as rocked bottom pond. A question that would arise from this, Do you want to maintain a little more constantly, or be more lazy and taking up a clean-up project that's a MAJOR ordeal once every 1-3 years, depending on leaves getting in the pond, fish load, plant's dying back, feeding, small insects, Etc.
In this type of set up, Bottom drains are pretty much useless, So basic filtration is pretty much your primary option. More complex filtration like vortex's, sieves, bead filtration, moving bed filters, and shower filtration now for the most part pretty much are going to be non-existent. Not that they can't be used, but rarely are, As the goal is keeping the pond as natural looking as possible, and not high tech goodies being viewed or seen in the set-up.
IMO, Koi should not be considered in the type of set-up shown, Especially high end Koi.( IMO High end Koi seem to be more susceptible to poor water conditions than flock spawn Koi, That's a discussion for another thread. LOL) Koi are going to dig, plants are not going to flourish as well, due to being Dug up and the leaves eaten, as the build up in the bottom increases the more likely the koi are going to stir things up (1 to 3 years down the road) and an unexpected gas releases are going to start the chain of events that leads to the big breakdown, First algae bloom, then a fish gets sick, medications are used, a fish dies and the whole enclosed system breaks down. Goldfish are far more suited for this style of pond. IMO.
Year's ago, there was much debate when salt water fish keepers started trying to keep corals and reef inhabitants. I said back then I thought there was more to the filtration and breakdown process than we may think, as the years progressed advances were made from both scientific and trial and errors and there were more discoveries of what and how plants used nitrates, anaerobic and aerobic bacteria became more discussed. Etc. etc. I still believe that there is still more to be learned and there are things that will surface in the next few years as more people start using bogs and rock bottom ponds with different degrees of success.
My advise to anyone that is going to try and take this route be open minded to what can take place and make plans for how and when you need to make adjustments, Plan before hand how your set-up is designed and read, read, read. Doing a pond with rocks has it pro's and con's. My advise is you need to know as much as possible about water quality, and how the breakdown process works long before attempting.
I personally don't agree with the keeping Koi, other than that, I do believe a pond can and are successfully kept going this route, and if properly done can be quite beautiful. But they can also be the most challenging and should not be done by novices, Experienced pond maintaining should be done before attempting this. Probably the reason you see Pond professionals promoting this is they are more guaranteed of return work, and they are more successful due to their long term exposure and experiences in the hobby.