The only time an O2 depletion would come into account with carbon dosing would be when its at its peak of eliminating the high end nitrates, it would not occur during normal daily maintenance doses.They all end up in the carbon cycle regardless of the method of introduction, just like in our tanks....and I think this (a jacked up carbon cycle) is where the problems are in both cases....tho there's still a lot to learn.
Maybe I'm wrong and it's all good! LOL.
-Matt