Hi, Flora (or Frolicsome),

I have 3 soil-based tanks set up at the present time. One is well into its third year. A fourth one is in process.

I use the least expensive topsoil that I can find. It should be low in organics with no additives. Before I use it I soak it for about fortnight, stirring and changing the water occasionally, to get rid of whatever "junk" might be in the soil.

I cover it with an inch or so of 2mm to 3mm quartz or silica gravel. Here that is called "blasting sand/gravel" or "builders' sand/gravel." The purpose of the gravel is to keep the soil where it belongs and out of the water column, as well as to provide something for the plants to root in. Pea gravel, if the grains are the size of a pea, might be too coarse.

Good luck!

Bill