Do you have any photos of your tank? That will help the most. The gas you're smelling is due to decaying matter in the substrate. Tell us about your tank--your filter, water supply, measurements (N, P, pH, KH, GH), fish, heck...tell us everything in as much detail as you possibly can. The bottom line is you should not smell anything from your tank. Something is wrong, so we need to fix it. Help us help you!
PS - You don't need special chemicals...no additives (aside from a tap water dechlorinator if you have fish) or anything of the sort (plant nutrients excluded). Methodology can fix dang near anything.