actually, if the media has been in the main tank then it is all set to go. Just take some of the big tanks media (enough to fill the requirements for the new filter) and place it in the filter for the small tank and fish can be added instantly. Make sure your main tank still has enough media though as you don't want to start having ammonia probs in the big tank. If you want too, always have a small filter running on the big tank, and when you need the little tank, you can just move the small filter to the small tank and your ready to go. This is ideal when dealing with a quarentine or fry situation