Matak--it may be a late reaction to the blackout. It is possible that the water under the filter plate collected some toxins while it was just sitting there. Young fish are typically more sensitive to these toxins than adult fish, and got a stronger 'dose' when the filters kicked back on. Same thing that happens in a box filter that is unplugged--without the water movement, the bacteria slowly suffocate.
Anyway, I would consider the loss of just one fish a blessing, when you consider the number of people that lost their entire tanks to the black out. I know it sucks, but it could have been much, much worse.