New tank experiment with deep sand bed

Hi guys, I just got a 29 gallon and am considering trying out a deep sand bed for filtration. I figure 3-4 inches of play sand topped with a layer of lava rock, plants and mts. I've read about the various "benefits" of having aerobic, anaerobic and anoxic bacteria. Some of my questions are will the roots and mts extend too deep and ruin my other layers, I think that I will only have problems with the area around the roots. Will the mts be able to penetrate the lava rock? I chose lava rock because it is porous and is too big for fish to dig up (hopefully). The only things I plan on including are lights (48 watts) and a powerhead angled toward the lava rocks. As for as fish, that I'm still debating on. Don't worry I will be testing every three days and have other tanks to move them to if I notice a problem. BTW the sand is coming from an established tank that has bee running for almost 2 years. Thanks in advance for any info and helpful comments, whether critical or favorable.