I"d have to disagree with that, Coyne and Mebbid. I've had, to my sorrow, a 'swamp gas' condition develop in my tank, and it killed a number of shrimp, some snails and some cories.
The water did not smell, but bubbles of gas were coming up from the substrate periodically and those that died were caught in the bubbles.
When I went to clean it, a huge cloud of bubbles came up, and the tank reeked of rotten eggs, sulphur dioxide. Eco complete, about 2.75 inches deep. Turned out I was overfeeding, and too much of it was sinking into the substrate. It's one reason I keep MTS now, they do help, but they only dig about an inch down, not any more than that, so they don't get to th bottom of thicker substrates.