Yeah, definitely not ammonia or nitrite. I tested the water and both were still 0. I've left filters off for a week, heaters off for weeks, etc without any issues. I'm sure a moderate to lightly stocked tank would cycle fine without a filter at all, just a powerhead or two for circulation. So I don't believe it's due to lack of filtration. I think once the bacteria in the filter use up all the oxygen they become anaerobic and start producing more toxic byproducts than nitrates. Which build up in the filter and then are dumped into the tank when the power comes back on.
Now when I have a power outage I just unplug the filter after an hour so it doesn't come back on when the power does and give it a quick rinse mostly just to dump the water out. No problems since I started doing that. So... :dunno: