$1 a gallon sale!
There's a lot of stuff out there about how to dirt your tank, but I went as simple as possible. I dug up some topsoil from my backyard (in Idaho). Some people say to put clay down, then other stuff, then MiracleGro (never with fertilizer in it though-always organic). I'm sure that works better than just plain old dirt, and maybe the nutrients will run out in a year or two, but we'll see.