I'm not sure what you are asking about the carbon. The diatoms aren't hurting anything except the tank's appearance. They are actually helping remove wastes. As overfeeding may be contributing to the bioload, you want to make sure that your fish eat everything you give them w/i a few minutes. As everything gets more stable, and your cycle completes, the diatomes should go away on their own. If not, you either need to feed less or do more frequent water changes. IME, as long as you equalize temp, and condition properly, frequent water changes are less stressful than large water changes. Do they add sulfur to your city water? I know some places use it instead of or along with chlorine, and I'm not sure what effect it would have on fish; I imagine it wouldn't help.