That duckponics thread is great, but I think that the author said that it did not work in the end. You need a very big biofilter to cope with duck pond water. With a big, well designed filter, it can work.
You would probably not need to run the pump continuously. You can just put it on a timer, maybe draining the pond once a day, with the pond refiling from the growbed/biofilter/reedbad via gravity. That would work fine.
One issue with a duck pond is that it will accumulate a lot of sludge on the bottom. Ideally, you'd have a hose on a timer, pointed into the pond, and just before the drain cycle starts, run water in to the pond to stir up the muck so a lot of it drains away. Then, when the drain cycle is nearing the end, have it squirt water in again to wash out more muck towards the pump.
If you are going to use a sump pump then I woudl not put it in the pond itself but in an actual sump that has the same height as your pond (so the water levels are the same) but that also is deeper than your pond (so it can completely drain the pond). The sump would be connected to your pond via a pipe running from the bottom of the pond and the sump. When the pump comes on it will drain all the water out of the pond and most of the water out of the sump (a pump can't drain right to the bottom, so you'd still have an inch or two of water in the bottom of the sump). The pic below shows what I mean. The biofilter is not meant to be floating in the air, just somewhere a bit uphill from the pond. You'd need the biofilter to drain slowly into the pond, and only when water coming in from the sump pump 'pushes' clean water out the other end. Also, you wouldn't actually need the 'U' bend in the drain from the pond to the sump....you'd just need a drain coming out of the bottom of the pond but then going into the side of the sump at a level lower than the bottom of the pond. Anyhow, here is the picture
You would probably not need to run the pump continuously. You can just put it on a timer, maybe draining the pond once a day, with the pond refiling from the growbed/biofilter/reedbad via gravity. That would work fine.
One issue with a duck pond is that it will accumulate a lot of sludge on the bottom. Ideally, you'd have a hose on a timer, pointed into the pond, and just before the drain cycle starts, run water in to the pond to stir up the muck so a lot of it drains away. Then, when the drain cycle is nearing the end, have it squirt water in again to wash out more muck towards the pump.
If you are going to use a sump pump then I woudl not put it in the pond itself but in an actual sump that has the same height as your pond (so the water levels are the same) but that also is deeper than your pond (so it can completely drain the pond). The sump would be connected to your pond via a pipe running from the bottom of the pond and the sump. When the pump comes on it will drain all the water out of the pond and most of the water out of the sump (a pump can't drain right to the bottom, so you'd still have an inch or two of water in the bottom of the sump). The pic below shows what I mean. The biofilter is not meant to be floating in the air, just somewhere a bit uphill from the pond. You'd need the biofilter to drain slowly into the pond, and only when water coming in from the sump pump 'pushes' clean water out the other end. Also, you wouldn't actually need the 'U' bend in the drain from the pond to the sump....you'd just need a drain coming out of the bottom of the pond but then going into the side of the sump at a level lower than the bottom of the pond. Anyhow, here is the picture