Thanks for your reply. I will look to find out what a big type filter is. Sorry for my ignorance but I will learn.
it's really just a natural and very efficient way to filter a water source. Instead of relying on smaller, more maintenance man-made constructs, you're basically providing a lot more area for the good bacteria to colonize and giving a lot of space for plants to be more involved with keeping your water clear. More plants means more competition for algae. More surface area (bog stone) means more good bacteria. With the filters you buy off the shelf, they require regular maintenance, especially if you're overloaded (and they more often fail at this point) with fish. A bog is something you only thin (the plants) on occasion as you want the aggressive growth of said same to work for you. A bog is more labor (sometimes cost) to set up but once you have, you'll do next to nothing for years. It also becomes part of your pond instead of something you have to hide or is a visible mishmash of drums, pipes, valves, containers, electrical panels, and various media, all of which cost considerably as you ramp up your filter needs to meet your pond's bio and mechanical load production. A bog simplifies all this and results in clear water. A bog is a biofilter, though, not meant to mechanical filter, though it does this too on some scale. Best to always put your pond pump up off the bottom though as you don't want to deliberately send large debris to your bog. Some have prefilters for just that.
In the 10 years now that I've had a pond, I only ever had green water initially and for barely a month as the system takes hold. And where you hear stories of green water every spring, I never get that, not since that first time. There's a simple beauty to a bog and that's why many of us here are Bog Warriors!
