Use topsoil not potting mix. If you don't mineralize outside the tank first you will get a huge ammonia spike until the topsoil has been mineralized by the bacteria. Mineralizing essentially is letting the topsoil sit until the bacteria has removed all the organics and you are left with just the mineral components. People will also add potassium to the mineralized topsoil too then top with sand. Go over to plantedtank.net and research before you start tossing dirt in a tank though because you will avoid certain disasters.