Sounds like its too late, but this is what I have in my notes (not sure where I got the info now and haven't tried it - sorry so long:
"Emergency Power used to keep 3 fish tanks powered (and a small fan) for the 3-4 days we had no power. If you follow these tips you could theoretically run your tanks forever without using a wall socket.
What You Will Need
Battery - Odyssey 925 cranking amps, sealed, deep cycle, dry cell battery. Odyssey makes batteries for the military and recently made these available to the public. This battery is smaller in size than your car battery, but packs way more amperage. The reason I do not suggest a normal car battery is because it is dangerous to bring a normal car battery in the house. Do not do this!
Normal car batteries contain sulfuric acid and are prone to vapor leakage. Very dangerous! The Odyssey is a sealed, dry cell. Nothing will ever endanger you inside this battery unless you were to crack it open with an axe. There are other sealed, deep cycle batteries on the market, but none are as small or low price as this one. Also has lifetime warranty. If your budget permits I suggest buying two of these as it will allow you to keep your tanks running seamlessly if planned right.
Power Inverter - 400 Watt DC
Power Inverter. I use a Black & Decker model I purchased at Wal-Mart for $40. Has quiet built-in-fan. I suggest using a 400 watt inverter as they have 2 outlets available on them. I have used power inverters from other companies and they will basically all do the job, however I must admit the Black & Decker unit was definitely quieter and of better quality. Make sure you get one that comes with alligator clamps that can be clamped directly on the battery. When you get this unit open it, test it, and become familiar with it.
Extension Cords - I suggest having several available as some people (like me) have more than one tank, but not in the same room as each other. Remember you are no longer relying on wall sockets. Your power is going to come from the inverter and if you have multiple tanks you will have to run extension cords to reach the inverter (this is only if you are powering more than one tank).
The inverter is going to come with alligator clamps so you can attach it directly to your car battery. I suggest you find a location that is in the middle of all your tanks. Put down a towel or some thick cloth on the floor. Place the battery in the middle of this towel. This is just to keep the battery off the floor and moisture away from it. Place the positive and negative (red and black) alligator clamps from the inverter on the terminals of the battery and turn the unit on. If you have everything hooked up correctly the inverter should power up. If anything is wrong the inverter will sound an alert or simply not power up. You can now plug in your pumps to the inverter.
If you have a big tank with air and multiple pumps, I suggest using only one pump per tank while the power is out. This will allow you to keep the water moving for the most amount of time. You don’t need to keep your air pump going so long as your water is moving (circulation oxygenates water all by itself). On my 75 gallon I have a canister filter and an Emperor hang on tank. I keep just the Emperor running when the power goes out, as it not only circulates the water but also heavily oxygenates it due to its bio-wheel. By doing this your fish can basically go on happily and never notice a change in the water chemistry.
Here is how you can keep this system running indefinitely with no wall power. The Odyssey battery is really a super car battery. If you purchased two you could have one powering your tanks and keep the other, you guessed it, under your car hood! With one functioning as your car battery it will always be full. When the power in your home goes out and you've used the power in your emergency battery just swap it with the battery under your hood.
Since the battery you are putting under your hood will be dead once you swap it, you will need a jump start to get your car started, and then run it to recharge the battery. You could do this forever and always have a battery full to run your tanks. I know this is a lot of work, but hey you only have to do it in times of extreme weather/power outages. "