A UPS isn't going to do you much good in this case. Assuming a 500w worth of heaters and powerheads, you'd need a huge unit to handle 8 hours.
There is no UPS made that handles that kind of runtime out of the box, unless you go massive, into the thousands of VA range.... or using a smaller UPS with external battery arrays... either is impractical to get both pricewise and in the timeframe. That's why I don't advocate UPS units for tanks... any outage that lasts long enough to harm a tank will last far longer than most UPS units will give you, plus they have a nasty habit of nuisance tripping GFIs, which we should all have on our tanks.
Here are a few suggestions that might be implementable in your timeframe...
First, if your office is as such that you can get an extension cord to the parking lot, you can buy or rent a generator. Even one of the cheap knockoff units they sell at auto parts stores, harbor freight, etc will work to run heaters and pumps. Alternatively, you can purchase a large power inverter, of say 1000w capacity. Units this large are not always the easiest to come by, but stores like Best Buy, and Fry's might stock them. Units this large also generally need to hook directly to a car battery with cables, so if you can get a cord to your car, you can pop the hood, hook the inverter right up to your battery, and leave the engine idling. The car will idle all night on probably less than 1/4 tank. I have used both of these solutions before and both work outstandingly well.
What does your company's IT staff use for backup power? Perhaps your company already has a generator or long term UPS solution. If you can get a cord to an IT wiring closet or data room, IT guys can usually be bribed with mountain dew or burgers without much hassle. If you're one of those jerks who is always whining about their computer, or installing things they shouldn't, good luck though. (sorry... sore spot
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Plan to be with your tank the entire 8 hours. You'll need to keep an eye on things and take emergency measures if necessary. Chances are this time of year you won't have issues with heat, but you may want to have some plastic bottles like milk jugs. You can fill them with hot water from a restroom sink and float them in the tank to help keep it warm. A cordless drill with a paint stirrer works well to keep your water oxygenated, just using it a few times an hour for a few minutes helps tremendously, just don't hit the fish.
Do any of those options sound appealing?
) If the outage was something serious for a longer period I have a generator I can 'borrow' to power the lights for a short duration.