Hello Charlotte,
Many questions...let me try to give some ideas and answers.
1. Size of the bog: After having built my pond and bogs (2 of them), I can only say that bigger is better! Are my bogs 30% of the surface of the pond? No, not by a long shot, 10% at the most, but they are keeping my water crystal clear with hundreds of large and small fish in the pond itself.
A bog is not only filtration, it's also competing with your fish for being the best part of the pond if you plant it well, I guarantee. With the pond filled with fish, your plants in the bog fill grow at an amazing rate.
2. How to set it up? For me the easiest way is to have the bottom drain connected straight to a settling tank. Settling Tank can be a 200 liter drum buried in the ground with the top of it about 10 inch above the pond's water-level. The water in the pond and the tank will be at the same level of course because of the drain going into the settling tank. The tank is necessary to have a place for the pump.
It also serves a purpose of large debris settling on the bottom of it, (if you use a round drum and the drain comes in under an angle, it will make the water start spinning. This will draw down heavy and large particles/debris). This is good, because less rubbish will enter and clog your bog area. Less maintenance means more time to enjoy your pond!
You can either use a bilge-pump or a bottom drain to take out this rubbish every now and then.
Hang a pump in the settling tank, around the halfway mark of water-level, connect it with a hose to the outlet pipe inside the bog.
The bog itself should be around 20 inch deep. On the bottom you put a 4" diameter pipe with slots cut (left and right side of the pipe, Not TOP and BOTTOM!) into it over its entire length.
Maximum size of bog area effectively served with 1 pipe is about 20" on both sides of the outlet pipe.
When you switch on the pump in the settling tank, water sprays out evenly from all the slots and rises up until it reaches overflow-point to spill back into the main pond.
Fill the bog with Pea-gravel or any other small 2" size stones to about 2/3 of the water-level where it would start overflowing.
Buy plants, space them a good 30cm apart and fill the space between the plants with more gravel until you are level with the top of the clay/soil around the plants.
Make sure you remove all pots from the roots or the bog does not work well. (if you need to winter your plants, consider nylon nets around it.)
Start the pump, give it about 4 weeks and you'll see how the plants start feasting on the fish-waste.
There's no need for any additional filters if you plan on a big enough bog-area.
You can connect both drains to the same Settling Tank, making the centrifuge effect bigger, or you can separate them into 2 Settling Tanks, in case of cleaning and for having 2 separate pumps/tanks for backup (one fails, the other one keeps feeding the bog's bacteria and plants and your fish keep getting fresh water. It's what I did as well)
Make sure you have a screw-cap on the far end of the Bog's PVC pipe. This makes for easy cleaning of the pipe if and when necessary.
Have a look at my 'Arapaima Pond' for many pictures and more details on what I described above.
Good luck, let me know if you need any details or answers.
Cheers,
Luc
