use miracle grow organic choice ( it dosnt have any ferts added) thats what most people use, you will take this and make about a one inch layer in the bottom of the tank (do all of this before adding water) then use whatever substrate that you wanted to cover the soil like sand or small pea gravel, you dont want to make this layer very thick i think not much more than an inch. you can then plant the tank or i waited til my tank was filled, then fill it very slowly with water so that you dont disturb the substrate too much. the reason for the soil under the substrate is to replace co2 and other ferts that you would normally have to add to a planted tank, over time the soil is suppost to break down and create co2.