Glaneon said:
I suppose.. if you're drilling for natural gas, etc.
I think you're missing the point. The whole theory is that if your sand bed is too deep, then whatever oxygen is down there gets used up by the first growth of aerobic bacteria. Without stirring the deeper sand, oxygen from the water I guess doesn't make it down there, so anaerobic bacteria can start to grow. Once they do, they start to convert nitrate into nitrogen gas. The nitrogen gas isn't toxic and just bubbles harmlessly to the surface. The gas pocket in the sand isn't whats toxic - part of the anaerobic process that converts nitrate into nitrogen gas is a conversion back into nitrIte. So, if one of these anaerobic pockets forms and then gets disturbed, you see nitrogen gas bubble to the surface, assume you hit a toxic "gas pocket", and any fish nearby can be exposed to a locally high concentration of nitrite until it gets used up by the aerobic bacteria in the filters, etc.
So there you go. I don't worry about any of that, but I did research the heck out of it at one point and thats the info I managed to retain. Little more to it than "anaerobic means no air". There is no air in the tank water, but plenty of oxygen, which is why the aerobic bacteria grows in the first place 8)