All the "pores" in lava rock will eventually fill up and it will cease to
work for bio filtration. I only know of three ways to clean the stuff: a)
lay it all out in the driveway and power wash it; or b) build a fire under
it and cook the "stuff" out. Of course, that option smells something fierce,
but it will work, or c) isolate the filter from the rest of the pond and
nuke it with potassium permanganate. That will eat the crud out, and once it
expends itself (or you neutralize it), it's harmless to your pond, plants
and fish.

Lee


"Tom Wikoff" wrote in message
...
With winter in west-central Ohio on the wane, I am nearly ready to
begin
getting the pond back up and running. This will be the second full year
for the pond. It is approximately 1800 gal with a 200 gal. bio-filter.
That bio-filter has approx. 50 bags of lava rock. Last summer a young
man
at the local nursery/water garden outlet advised me to take out and
replace
the lava rock because of my complaint about string algae. I had never
heard of the need to replace this rock and find the thought of the cost of
replacing it just a bit hard to swallow. Was that sound advice? Can't
the
rock be "cleaned" instead. NOTE: I do keep small bales of barley straw
in
the both the filter and at the waterfall and treat the water with products
recommended by the people who sold me the liner, pump and fish (5 to
date).
The ratio of time I spend maintaining versus enjoying my pond does not set
well with me. Any advice would be greatly appreciated.