lol! That's a bog, a haunted area of your pond!
I'm sure kelkat has the same definition, but a bog is a gravel and dirt area full of water, though not over the surface of the soil, where you can grow water plants to your hearts content and it will be full of frogs and toads!
What I've done and many people do, is filter the pond water through the bottom of the bog and the plants and bacteria do a heck of job eating up nutrients in the water that otherwise feed algae. It's the only filter i use and it has worked impressively for 4 years now, though my bog "pond" is quite large.
Some countries actually filter their sewage through fields of "bogs"/wetlands, that's where i got interested years ago when i saw that on a show.