Hi Nathan - Yes, you have the right idea, and it would work, it just isn't
large enough for the size of pond you have - the denser the media you use,
the more surface area you have for "good bacteria" to live on. The only
problem with this is, the denser it is, the quicker it plugs up and no
longer allows your pond water to pass through it and has to go around it, so
you only have the outside surfaces left for your bacteria to biologically
filter the water. Sponge has the most surface area, but clogs up in a few
days and is hard to rinse - then you have Scotch type scouring pads (which I
use) - more porous and easier to clean and will run all season without
cleaning if the quantity is large enough - then all the rest (screening,
lava rock, pea stone, hair curlers, cut up pop bottles and anything else you
can think of that will give you enough surface area to host enough "good
bacteria" to "eat" the stuff in your pond water that causes algae bloom
and/or cloudy/green water. You can use a smaller container than the 45 gal
drum I use, but will probably have to clean it more often, but just remember
it takes time for a filter to become biologically active, so you can't clean
it too often - the longer it runs between cleanings, the better it works
I also try to run my pond volume through my filter/ hour even though I
keep reading once every 2 to 4 hours is good
Gale :~)

The reason I was looking at placing lava rock into the basin around the
pump is to provide a place for "good bacteria" to live. In other words I
wanted to create a small bio-filter next to the pump. Since lava rock --
or some other material (like the nylon scrubbers) allow for bacteria to
grown on them -- wouldn't this work?

Nasa