I have an extra 5.5 gal and a 20 gal tank in case i ever need them but they are in storage, no need to run them all the time (plus the bacteria would need to be fed or they'd starve, so you'd have to constantly dose the tank with ammonia to keep it cycled). I have a spare heater too. I have plenty of media from my cycled 40 gal (I run a canister, HOB and a spare corner sponge filter) so I can either pull the corner filter if I need it or grab some media from either of my filters. A small portion of media from your current filter would instantly cycle a smaller tank with one or two fish. It's also often a good idea to run a small spare filter on your main tank for just this situation.