I would Not replace the lava rock. No Way. Lava rock is
too heavy and just all round pain in the rear to deal with.

Better to build a Veggie Filter adjacent to your pond..
make the VF at least 10% of the surface area of your pond.
Don't put rocks in it at all. Just plants and more plants.
I just let the water do the work. I have a picture of my VF
on the first page of the first link below my name.

Nedra
http://www.geocities.com/Heartland/Pines/4836
http://community.webshots.com/user/nedra118

"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.