The air pumps Kiah is referring to are an emergency device that turns on when the power goes out (they have a plug to sense when power goes out), and they run on two D-cell batteries).
Salt water tanks need good exchange of oxygen and CO2 to keep the fish alive, and usually there's plenty of that from the skimmer, overflows, and turbulent surface water. But when all those stop in an outage, you really do need to get air into the water. That's the primary concern. You can do it with an emergency bubbler and/or a powerhead on a UPS aimed at the surface.
For about $10 each, I personally think the auto-air pumps are required equipment for any reef tank.
We did a group buy a while back and they were $7. Might be worth doing one again if enough people are interested. HeavyDC was the organizer last time.