I've gone thru this before with "Snowmageddon" a few years back, so I'll share my advice.
First, when the power goes off unplug everything in the tank. There are a few reasons for this 1) the bacteria in your filter may die from lack of oxygen and if it does you don't want it to suddenly turn on and spew all that dead stuff into the tank and foul the water 2) you may forget that the light is actually still turned on (since it appears off) and if you cover the tank and the light suddenly comes on it can overheat the tank quickly or even start a fire (this happened to me and I nearly cooked my fish). So just unplug everything as soon as the power fails.
The best way to keep the tank warm is to stop the heat from escaping in the first place, so cover the tank right away. I use minibow tanks, which are kind of tall, so I just slip a pillow case over them and then wrap in a towel. I've even used a shirt before in an emergency. Try to cover the entire thing so no heat can escape. If the power is out for many days you'll need to uncover the tank for a few minutes each day to replenish the oxygen supply and do a quick water change. If you use Prime as a water conditioner you can use a bit more to help with ammonia. Also keep in mind that the cooler the water is the slower toxins will build up, so if you are able to keep the water warm you may still want to keep it a tiny bit lower than usual. Also, do NOT feed the fish during the power outage at all, the less he eats the less waste he'll produce and the less toxins in the water. Your fish can live at least 2 weeks with no food, so a few days won't hurt him.
If the hot water heater in your house is gas you may still have hot water - if so you can add dechlorinated warm water to the tank if it starts to cool down. It doesn't take much warm water to jack the temp up in a small tank, so don't make it too hot and only add a bit at a time and keep an eye on your thermometer.
If you have a gas stove you should still be able to use it as long as you have a match or lighter to light the burners (kids - get your parents to do this DON'T do it yourself). Since the pan you use might be contaminated it's best if you heat up a pan of water and then set a jug of dechlorinated water inside of it to get warm. If you don't have a jug or other clean container you can even put some dechlored water in a baggie and float it in the pan of hot water till it's warm.
If you have no way to heat any water then just keep the tank covered - the temp will drop slowly so it shouldn't be too much of a shock to the fish. If there's nothing else you can do then just keep it covered and hope for the best.
Once the power comes back on, rinse your filter media out in some dechlored water to help clean out any dead bacteria. Keep an eye on your ammonia levels for the next week because you could go thru a minicycle if you lost alot of bacteria.
I'm preparing for this hurricane myself - I have 3 betta tanks and a 40g tank. I have all my supplies - blankets, towels, jugs of dechlored water and 2 battery operated air pumps (for the big tank) and a flashlight sitting on a chair in my dining room so that if the power goes out I have everything in one place and ready to go and I don't have to go searching for it if it's pitch dark in the house.
Last edited by Wendyjo; 10-28-2012 at 08:33 PM.