John....I use lava rock in milk crates.... I think you do NOT want to use any
soil. You want the roots to escape as soon and as far as possible to "eat" the
stuff from the fish pond..Remember I run a 2500 gallon goldfish pond, and a
10,000 gallon koi pond with no other filters....Jerri

http://www.fringeweb.com/Ponds/JerrisPond