Hi, my pond that is under construction is 9'x12'x20" and I am building a 3'x3'x18" bog to use as a bio-filter and to feed the waterfall. I have been told by a pond-shop owner that I should use lava rock. I am also wondering about the information in this forum that talks about the frequency that the mechanical filter has to be cleaned if you have a bog-filter. It seems to make sense that the less algae in the pond due to the additional bio-filter the less cleaning of the mechanical filter you would have to do. I also have a thread started under ponds in this forum with some good information given.

Lava rock is a horrible medium. It starts out good and then all the tiny holes fill up with junk and all you have left is big rough rocks.
Pea gravel and even large grain sand is good media for a bog filter because you aren't ever going to tear a bog filter apart and clean it. It's permanent.

With lava rock you will have to replace the rock.

You don't put any soil in the bog filter because it will just wash out and make your pond dirty. All the nutrients for the plants in the bog filter come from the water and the sand/pea gravel serves the purpose of holding the plants in place.

I'd suggest you add some plants that will survive your winter so you don't have bare rocks several months out of the year.
Add pennywort, water celery and watercress. They will keep your filter green and functioning for most of the year because they thrive during different seasons that the other plants.

Cardinal flower will have to be replanted every year because it won't survive your climate. Tomatoes and Impatiens are good summer plants for the bog.

Pea Gravel is the way to go. Rinse it off first. pennywort is okay but if you are going to have other plants in there they are a hog and you have to constantly cut them back which makes them grow even faster! I got rid of all of mine in the small pond. Just too much work too control their rooting system (because ya cant *lol*).

If you do put clay soil in the bog filter do not put more than an inch on the bottom and cover it all up with pea gravel assuming it is a fairly decent size. Anything more than this will make your water cloudy and dirty looking. You will have to start all over and you dont really want to do that. Well maybe you do but I hated it! *S* I suggest leaving out the soil completely because any plant you have in there will get their agrofom tabs and all the yummy stuff filtering up through it to feed your plants. (and this is THE only dirt I suggest using for any aquatic plant. Rule of thumb is the nastier the clay soil is the better your plants will thrive.)

The cardinal flower is a beautiful plant and well worth any replanting you might have to do though I do not suggest it in a bog of this size. When you plant in a smaller pond like you are doing, you want to keep your plants in it simple and plant ones that wont overrun your filter with a enormous root system. Irises are wonderful, as are aquatic mints (We grow chocolate mint in one of ours and wow it is nice smelling and very pretty). Just remember in a bog filter this size to keep the plants simple. Pickerel weed will look nice but I personally think it belongs in the pond itself or in a larger area. It just gets way too big for the bog filter of this size.

Hmm but in reading your post again you never did say how big the bog filter -would- be....can you tell us?

Here is a website that can maybe help you out:
http://www.fishpondinfo.com/marginal.htm then you can look around for pictures of the plants.