Dorkhedeos
Boss Major
well, we need a canadian area code
Someone actually tried a UPS before on a forum (might be this one) and they found that it didn't work very well at all.
It was on a reef, but the problem would be similar on a FW. The load that is seen on an impeller (like that of a powerhead, or filter) is different to that of a computer (I can't recall exactly, but I think the PH was a reactive load instead of an impedant load, though I may be way out with that). The bottom line is, when they tried it and cut the power, the pumps died too.
You may be able to use one for an airpump to ensure some circulation around the filter...