The B11's will easily run 12 hrs on a pair of D cells IIRC (I think longer actually).
One at each end of the tank will keep the water moving enough to keep the O2 level liveable indefinately IME. They won't heat the tank, but dropping O2 is the big killer. Air pumps may not save you if it's cold outside and you lose power for several days, but really nothing short of a generator is going to get you through that.
Jay would know far better than I, but IIRC UPSs usually don't last all that long if you have any significant draw on them.
Your other option is deep cycle marine batteries and a power inverter, but I'm not really framiliar with those set ups.