I don't know if you would really call this a bog but I do. It doesn't filter the water that I know of but I may be missing that it does because it is just at the end of my stream that goes into the pond and I don't see any effect one way or the other. I hope there are others that might know because I am also curious.
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Here is a link that might be useful: My pond.

Our pond is about 2,000 gallons & we have a bog for filtration. It's about 10 yrs old now & is beautiful & healthy. I'm pretty rusty about posting pictures here so if you'd like to see some just e-me. Bev

We too have a bog filter for our 10,00 gal. pond and I just posted some pictures this morning for a similar thread on the ponds forum. Hope something here is useful.
~Sherry

Here is a link that might be useful: Bog Filter