Dag-rowan wrote:
I would like to be able to slow down or shut down the growbeds in winter but keep the fish running all year if possible
This seems to be the key.
As far as additional filtration there are several factors that come into deciding what to use, stocking rates are one of those, whether a DWC or NFT is connected is another, and the other that comes to mind is wanting to shut down a systems grow beds for winter. I may have missed other reasons

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Sounds like you want to do high stocking rates and shut down the grow beds for the winter. Both of these will need separate filtration (other than the grow beds alone). Basically you setup a Recirculating Aquaculture System with the fish tanks and add a Radial Flow Filter (to remove settleable solids (RFF)) and a Static Upflow Filter (to remove suspended solids (SUF)) and for biofiltration you'd add a Moving Bed Bio-Filter (MBBR/MBBF). That would be the Recirculating Aquaculture side. This next thing needed is a Mineralizer. The solids come from the RFF and possibly from the SUF and go to the mineralizer where they are digested and broken down into nutrients that the plants can use. The liquid portion from the mineralizer goes to the plant side (perhaps to a plant sump) where it is recirculated by a separate pump in a loop all the way around the plant growing area. Because no water ever goes back to the fish side you can fertilize the plants separate or treat them if need be. I should also mention that you still could pump some of the water out of the fish side to the plants even if it doesn't go through the mineralizer - the one solid that wasn't removed was the dissolved solids which includes ions that are plant nutrients. I'd probably do that when it was time to replace more fish water than I needed just to flush solids to the mineralizer.
None of these filters is all that complicated and you can make your own. Sizing is a bit more guess work and depends on the stocking density and the media used.
You need to watch the carbonate hardness with a RAS setup because if it gets too low the pH can fluctuate rapidly.
Andreas (Dasboot) has a dual loop system (He has a lot going on right now and hasn't been checking in much lately but hopefully will at some point). I don't have a dual loop setup but I have a RAS setup and a separate aquaponic setup and there are others here that probably can help you if you need it.
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You could also waste a portion of the fish tank water every week to keep the water quality within your parameters or do a combination of filters, wasting, and grow beds so there are lots of choices.