Over the years I have set up quite alot of tanks including closing down a fully stocked Malawi tank measuring 72" x 24" x 12" just to change over to a 72" x 24" x 18" and had it up and running all in the same day. If you have ever found yourself having to set up a tank in a hurry for what ever reason then here are some tips to help get the good bacteria going fast. It is not always possible to keep the tank water so the next best thing is the muck from another tank which can be a friends.
Once your new tank is running clean out as many sponge media from any filters into the new tank or a container. However remember that the bacteria from the sponge can only last about an hour out of tank water so be quick when transporting the muck. When this goes into the new tank it will make the water look dirty but in time this will clear and the bacteria will find itself into the new filters. You can also take the dirty water from a friends tank when they do a water change.
Another tip is to drop in a small pinch of food, usually flake into the new tank every couple of days. This will encourage the different bacteria to start and also give them something to feed on before introducing the new fish.