There is a lot of mis-information, both in FW & SW, as to where O2 comes from. As stated on this thread, it comes from the exchange of gases at the water surface, period. (with some exception that someone will mention.) This is why surface area is almost as important as tank volume when it comes to "how many fish can I get". If fish can go a couple of days in a shipping bag; they can go quite a while with no help in a power outage. (even though the bag has pure O2). I think the biggest problem with a power outage is the loss of flow to the bio-filter bacteria (and subsequent bacteria die-off); but I could be missing something.