Originally Posted by Maxillius
Hi I am no expert but I did read up somewhere that bacteria in substrate could turn nitrate into nitrogen gas and escape out the surface of water this way! I'm not sure in what quantities or anything but I bet some other folks around here would be able to answer that :)
Usually it is Methane gas that is trapped in the substrate then released when vacuuming, this is caused by the Anaerobic bacteria in the substrate. Usually a good sign of over feeding if you see bubbles coming up when vacuuming.
Most will never cause problems unless allowed to build up to excess levels, which will be noticed by other means before they cause any harm, ie stressed fish and readings of ammonia.