I agree that the cannister alone is not enough circulation. I like the koralia circulation pumps.
What kind of dirt and what kind of cap? How did you prepare the dirt?
Running c02 and then suddenly stopping could be a culprit. Are you running a tank that requires c02 based on lighting and plant selection?
Next, I would consider the lights. As mentioned above, what is the photo period? Any idea what PAR you are sitting at? I completely disagree with the comment that you need to be at exactly 6500 to 6700k for your lights. This assertion would bust all those using cfls at 5000k and other temperatures . The important measurement is PAR.