I've found that if there's one tool that an aquariast needs in an emergency is a well seeded sponge filter. I put a few big ones (2x40 gal size) in the sump of my 75 gallon cichlid tank and have ended up using them more times than I can count. Power outage? Hook up the sponge filters to battery operated air pumps and drop them in the main tank. Fish have babies? Net the little suckers up and drop them in a 10 gal with tank water and a sponge filter. Voila: instantly seeded grow-out tank. Sick fish? Same story with a hospital tank. Because I have 2 and only in outtage situations do I need more than one, I can take the sick and medicated filter, sterilize it, and drop it back in the sump at my leisure. I rely on the water passing over the filter to keep them seeded. I'm sure I could hook up an air pump and keep them a little more effective, but this way they don't clog and they have plenty of benefical bacteria to start filtering when they're bubbling.
If you're not running a sump system, keeping a hospital/quarantine/grow-out tank with a sponge isn't a bad idea. You can keep it running with any hardy fish. No lights are necessary and in most instances, all you need is enough space to put a 10 gallon tank.
