Since you already have a tank running, if you get a canister filter, run it in your exiting tank for a few weeks to seed the bio media, or if you already have a cannister, swap some of the media into the new one when you run it, this way you will cut back on the cycling requirement for the new tank.
If you cannot do this, do not put all of your existing fish in the new tank all at once, as Whetu says, start it slowly.