I'm using a store bought brand. It was actually the most expensive potting mix I could find. Are you certain that the yellowing and eventual death of the bottom leaves is normal? I'm finding the next set of leaves starting to yellow up? Which particular strains do this? I'd like to find out more on this.

I have also added some more pictures, showing the entire plant. It isn't looking particularly strong due to over watering, however the dying leaves are clearly visible.

http://img818.imageshack.us/img818/784/problemwe.jpg