Abran, if I were you, I would stay away from lava rock or pumice as a filter material.
They are great fitering materials but they plug up too quickly and are difficult to clean properly.
Try nylon pot scrubbers, bird netting, PVC strapping or short pieces of corrugated PVC hose, floor scrubbing machine pads, Air conditioning cut-to-size filter pads, Polyester quilt batting,Etc.
Or,
commercially sold Bio-Balls, Matlata Mat, Spring-Flo strapping, filter bristle brushes, Etc.

Remember the material you use will have to be cleaned at some point. So keep it simple. light weight and easy to clean with a stream of water from the garden hose.

A drain plug or a ball valve, located at the bottom of your filter container, is a handy thing to have when you do a clean out. Make it 1 1/2" or 2", at least, so as it lets the sludge run out, without plugging up the drain.

The sides of your pond will eventually become covered with nitrifying bacteria, the same bacteria that will colonize your filter material and convert the ammonia, which produces toxins [nitrites], from fish waste and decomposing plant matter, into less toxic food [nitrates], that in turn, will be used by the plants in your pond.
"Horton"

Here is a link that might be useful: Homemade filters

Abran,
The question has to be what size is your container and size of the pond?
Mike

Abran, the bio media I used is shown in the link below. It's a little expensive, but you can make your own with scrubbies and a mesh laundry type bag.

Like Horton said, I would stay away from the lava rock. They're incredibly heavy, and nearly everyone I know who has started out with it has switched to something lighter and easier to clean. The bag of scrubbies is great because you just agitate the bag in the water to loosen the debris and then drain off the dirty water.

I made my external bio filter out of a plain, ordinary trash 32 gal trash can, but any type of suitably sized container will work. The pump is in the bottom of the pond, and the water flows through the hose to a swing type check valve. On the other side of that is a T joint with one side leading into the trash can, the other leading to a ball valve which enables me to backflush and drain the dirty water whenever I rinse the bio media.

I made a low frame out of pvc for the bio media to sit on so it won't block the intake hole, and then a larger hose near the top of the trash can carries the water to a small bog where I have planted some taro. From there, it drains directly into the pond.

Here is a link that might be useful: bio media

My filter is based on the Skippy filter design. I used Matala filter mat in different grades, starting with the coarsest at the bottom. This stuff back washes very easily, I have twelve fish in a bit over 400 gal. and have never had water quality issues. I have been using it for the last three years. Expensive, but well worth it in my opinion.

Here is a link that might be useful: Matala filter mat

I have a square mesh planting basket full of plastic filter mat that is similar in texture to scrubbies used as a prefilter to my pump and it has kept my 900 gallon fish pond clear for years. (DH works in heating and cooling, I have no idea what the mat stuff is really called. It is blue) Until a kingfisher got them all two years ago I had about 40 goldfish, some ten years old.

Used to have lava rock but it is too dang heavy to lift out and rinse off. Used a bucket with holes drilled in it with the pump inside and lava surrounding it for years though. It worked.

I do have many plants. I have multiple small ponds with no filters at all that are always clear.

Hi Abran- I DIY'ed a filter for my 2k gall pond, and it works exactly like it should. I have a few pics on the thread below- instead of re-posting here, I'll just direct you to the site.

Hope this helps...

Here is a link that might be useful: My DIY Filter