hydrogen sulfide pockets

Hydrogen sulfide is one of the by products from anaerobic bacteria processing nitrates. It will bubble out of the tank without harm after being processed by yet another layer of bacteria. A crew of sand movers--worms, some snails, stars and other critters--will prevent the bed from becoming anoxic (no oxygen, instead of low oxygen). Major disturbances can result in a mass release, but this only happens when you have an established bed that doesn't have adequate sifters and something is added that shifts huge amounts of sand very quickly.

I've disturbed the entire sand bed without having a problem, though, so my feeling is that it's pretty rare that it's a problem.