Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The bacteria in the filter need oxygen. Open the filter, and spread the media in a shallow tray that can keep it damp, but has lots of surface area for gas exchange. It does not have to be under water, just damp. Turn the media over often, stir the loose materials.
The fish need oxygen, too. With no water movement all the gas exchange is happening at the water surface. If the fish are not up at the top of the tank, then they probably have enough oxygen. You could stir the water some by simply scooping it up and pouring it back into the tank, but water changes are better. You can let the refill water pour in, making some splashing. You do not need to do large water changes, the real reason to do them at all is to get more surface movement.
Fishermen use a battery powered air pump that runs off 12v, and there are battery powered air pumps that you would keep plugged into the wall. When the power quits they automatically switch over to battery power. Either of these options would keep the water turning over in the tank enough to keep any reasonable fish load healthy.
An overstocked tank might need more extreme help:
Do a 50% water change, saving the aquarium water in a plastic storage bin. Add new water to this (and to the tank, of course). Move several of the fish, up to half of them, to the storage bin. Having less fish in the tank makes it easier to handle a power outage. Of course you then have the equivalent of two tanks to deal with, but each one will not get into trouble as fast as one over stocked tank would have.
In the dark (or at least with the tank lights off) the plants will probably not be photosynthesizing, so turn off CO2. If the tank is close enough to a window that it gets natural light, then the plants will likely be photosynthesizing during the brightest part of the day, so you might want to alter the timing on the CO2.