Rotting

Hi guys, I set up a little +- 30l tank with a large piece of bogwood and, thinking I'd be clever, I used quite a lot of pottery clay for a substrate, capped, although I don't think very effectively, with some riversand. Put in some plants, not sure of names, but ones that were doing well in my other tankwith just riversand. The plants didn't thrive, leaves stareted falling away, and today, about a week after original setup, I took them out and they STANK of rot, smelled like a sewer. Why were they rotting? I think it would be the hectic amounts of clay, they certainly get enough sunlight. Could the clay have gone anaerobic, and is that a problem?

Thanks, Gabriel