I think I figured out where the bacteria came from. although i'm sure i've had zero oxygen pockets for a while now, they have only recently started to form. I figure this is because I have just begun supplementing nitrates for my plants, as I noticed the bubbles a week ago, just as I was beginning to supplement my tank. I got all the gas stuck in the gravel out last night, and gave those nasty bacs a huge blast of deadly oxygen, so I suppose all i need to do is stop adding nitrates to the tank and this problem will be done with. although I have never done an official gravel vac, at each PWC I stir up parts of the gravel so the crap goes into the water column, and then suck it all up with my siphon hose.