I always keep tanks running even when I don't have anything in it. I keep a feeder guppy or two or whatever small fish that I have at the time to keep the biofilter going.
When setting up a new tank, I would take off 2 filters from 2 tanks, put it onto the new tank, and put on the new filter. With 3 filters running, I rotate the sponges between the three filters every week or so to get the bacteria happy.
In the meantime, the other tanks, which still hold the feeder guppies, are still ok unfiltered (I keep my tanks bare bottomed for easier cleaning) since it only has a couple feeder guppies.
Some people swear by bio-spira, some say it doesn't work.
I think it is because biospira must be kept at a certain temperature to stay dormant and that is pretty hard to do. If it's too cold, it will freeze and kill the bacteria. If it's not cold enough, the bacteria will become active, use up any food they have, then die of starvation.
Most cycling products such as bio-zyme and cycle don't really work. The fastest way would to put a feeder goldfish or two per 10 gallons of water depending on the size of the goldfish. The bioload will get the tank cycled faster, but will be off to a shaky start once you raise the temperatures or remove the goldfish and add other fish.
Eric - Secretary/Bowl Show Chairman