I have never had an outage that long I run a UPS battery on my tanks , it runs just the pumps , since the pumps put out heat it helps keep it warmer . The battery lasts on our 10 gallon aprox 12 hours or so ... I need to take the tank down and let the battery drain and recharge every now and again so that I can make sure the battery is at full capasity , the down side to that is if you do that with a battery , and your power goes before it is fully charged
However I have had my tank drop due to a heater malfunction and when I got the extra heater out (I always have a spare ) I brought it up about 1-2* an hour then let it do it the rest of the way once I hit 71* in the tank .. I had some ticked corals but every thing did fine... The temp hit 65* on mine so not much less than yours ..
Next time think about wrapping the tank in a couple blankets to insulate it a bit if possible