Well I have to agree with heather you dont have to buy the bacteria a simple setup like she said will make a nice colony. Also if you have any bodys of water around you (ponds or lakes) you can collect a colony from the shallows, then quarantine the colony and do your water changes. I personaly like lake bacteria because its so much more well developed. In my years of doing fish tanks I always used sand (silica sand "play sand" from any hardwear store), still do and the bacteria seem to do very well in it. About putting it under the substrate, im not sure how well it work but in theory it should work as long as you keep the colony submerged in water. When you figure it out let us know and keep us posted