Any chance you could go to a veggie filter of some sort? We moved
away from lava rock because it got clogged so fast that it seemed
functionally to be just rocks! Screen has lots of surface and still
let water through. Mostly, however, we have gone to a veggie filter
because the roots are such good surfaces and pull nutrients as well.
The slow flow through and the roots pull out most of the solids as
well, providing mechanical filtration.

Jim