JGW wrote:

We're getting ready to build our new pond. The contractor wants to
line the walls and bottom with rocks, which he says will serve as a
great huge biofilter. I have read that it's impossible to keep the
pond clean with rocks on the bottom, and that they can trap hydrogen
sulfide gas.


There is a recent thread on koiphen discussing this:
http://www.koiphen.com/forums/showthread.php?t=23193

A good points made about rocks it that they will eventually get covered
in biofilm and it is had to tell that they are rocks.

Rocks trap all sorts of bad stuff underneath them. It's just not a good
idea for a closed system like our ponds. If you just had to have rocks
I would make sure they were mortared in so that nothing could get in
cracks or crevices. Don't just set them on the bottom for sure.

Mark B.