I originally was going to use egg crates to help build under the hills but I've decided on just using the plethora of small lava rocks we have in the backyard. We use to use them as part our Koi Pond filter system, though upgraded to bio balls last summer. They were washed upon leaving the Koi Pond, but have just sat outside around the waterfall the rest of the time.
What's the best way to go about cleaning them?
If not I'll probably just go to Lowes as they have small chunks of lava rock in bulk real cheap, but if I can reuse them, that would be awesome.