Hi Dave,
Re sinking the RDF into the ground; Nah, the filters live in the garage and it's a concrete floor - I'm not chipping out any of that just for the Oase when there's others that do the job equally well. As it happens, I sent an email earlier this week to Select Nishikigoi Intl and got a reply from Paul who sent me the installation manual for the BD300. I've also been in touch with Mike from QuiniKoi about their QK22 and Tony from DracoDrum asking him when their new standalone unit is due for release (mid May it looks like). I'm still torn between them as I haven't seen one (of these models) up close (can't find a local stockist near me and I don't know anyone personally who's got one) so I'm researching at arms length and relying on t'internet forums and YouTube which is not exactly the most reliable way to judge how you spend the best part of two grand but it is what it is.
I can't remember if I've explained my set-up previously, forgive me for repeating it now if I have. I've got a small, unheated, back-yard pond averaging a fraction over 3ft deep and currently holding 1417 gallons (if I've done my sums properly). I've got a low-end Oase pump (rated around 3500l/h off the top of my head) that feeds a small stream feature that incorporates an alfagrog bed and a few marginal plants to provide a small element of bio/veggie filtration (but mostly it stops dead holly bush leaves being washed into the pond). Needless to say, I've not got a skimmer

. The pond wall butts up against my garage so my current meagre filtration system consists of a small two-chamber ERIC type filter (16 brushes in the first chamber, layered jap mat and a final layer of fine filter matting in the second) gravity fed via 4" pipe from the bottom drain. There's another small Oase pump that pushes the filtered water through a 25W Yamitsu UV and back into the pond just under the surface with air provided by a venturi. I don't know the flow rate but it pushes a fair whack through and, along with the stream entering, generates enough of a current to gently circulate the water all the way around the pond. It's probably better if I show you what I'm on about so, on the premise that a picture paints a thousand words, here's a couple that hopefully helps puts the description above into some sort of context:

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Like I've said elsewhere previously; there's no prize winners in my pond. They're mostly low quality, garden-centre specimens but they're friendly and a couple of them are due for their fishy bus passes at 25 yrs old or so. My water parameters are surprisingly stable and have been since the filter matured in the autumn. pH = 7.5; NO2, NO3 and NH4 all 0 or as close to it as you can tell with the limitations of the test kit colour chart.
So why do I want to upgrade to a RDF and an anoxic filter if my water's ok as it is? Well, as small as it is, it takes me around 2-3 hrs each week to clean the filter and associated gubbins. That's time I could use doing other things to be frank! Another reason is the amount of fine particles that I have floating around makes the pond water look like soup when the lights are on. The third is that I worry the current filter won't be able to cope with much more of a load on it and I'd like to add one or two higher quality specimens to my collection in the future. Nothing big or too fancy, but I'd like to see them grow in time and I just think the anoxic filter is a simple and elegant solution which would also be easier and cheaper to adapt than installing something along the lines of a new Nexus or even a home-built K1 fluid bed system. The new frame is big enough to add another 233 gallons (again, assuming I've done my sums properly for a volume of water at 6' x 2.5' x 2.5') which I think will be big enough to take 20 stacked biocenosis baskets. I plan to pump the water from the RDF to the anoxic pool which I've raised so that the max level of water in the pool will be a smidge over 12" above the max water level in the pond (which I hope will be high enough to generate a reasonable flow under it's own pressure back into the pond.
I haven't got to grips with "flow rates" yet. To be honest, I haven't a clue how these are calculated. I know I'll have a total of 7500 litres of water (pond, stream and filters) and this will need to be pushed through at least once every hour but I don't know how much to allow for loss due to pipes and their bends, length, width or height. Would that even be a consideration in my case? There are a number of pumps available that say they'll work up to 1650 gph which sounds about right (I'm thinking of the Oase Aquamax 8000 off the top of my head) but if I put a much bigger pump on, say something like 2900 gph which presumably pushes almost twice the volume of the pond through in that time, would that be too much, too fast? Will the water loiter in my anoxic filter long enough at that rate for the baskets to do their stuff? What is the ideal "flow rate"?
There's a lot still to learn about this hobby. I'd better stop rambling on and get some kip - maybe the answers will come to me in my sleep. Better still, you guys with all your accumulated knowledge and experience, might point me in the right direction? I live in hope. Best regards as always, Mick.