The most immediate danger in most cases of a power outage is temperature swings.
I figure five hours is better than carbon monoxide poisoning.
Five hours of pumps running won't be of any use to people who lose power for 24 hours.
If you choose to run a generator in your living room, or right outside your window, then yes it could make you sick. But if this is the case, natural selection probably would have done you in long before this.
But what do I know I only have eighteen years in the emergency services.
Enough to get your fish through a power outage. Assuming the outage is not longer than a day or two, and that the climate where you live is about 78 degrees, day and night. Year round.